Exploring Extension Blocks and Constants in C# 14

Extension blocks - Extension properties

Extension blocks and an important new feature - extension properties can be made in C#14. This is available with .NET 10.

It is not possible to define a generic extension block to add extension properties / members (yet, as of C#14 anyways -maybe for future version of C#..)

Consider this example of some well-known constants from entry-level Calculus using extension properties.

using System.Numerics;

namespace Csharp14NewFeatures
{
    /// <summary>
    /// Provides well-known mathematical constants for any numeric type using generic math.
    /// </summary>
    /// <typeparam name="T">A numeric type implementing INumber<T> (e.g., double, decimal, float).</typeparam>
    public static class MathConstants<T> where T : INumber<T>
    {
        /// <summary>π (Pi), ratio of a circle's circumference to its diameter.</summary>
        public static T Pi => T.CreateChecked(Math.PI);

        /// <summary>τ (Tau), equal to 2π. Represents one full turn in radians.</summary>
        public static T Tau => T.CreateChecked(2 * Math.PI);

        /// <summary>e (Euler's number), base of the natural logarithm.</summary>
        public static T E => T.CreateChecked(Math.E);

        /// <summary>φ (Phi), the golden ratio (1 + √5) / 2.</summary>
        public static T Phi => T.CreateChecked((1 + Math.Sqrt(5)) / 2);

        /// <summary>√2, square root of 2. Appears in geometry and trigonometry.</summary>
        public static T Sqrt2 => T.CreateChecked(Math.Sqrt(2));

        /// <summary>√3, square root of 3. Common in triangle geometry.</summary>
        public static T Sqrt3 => T.CreateChecked(Math.Sqrt(3));

        /// <summary>ln(2), natural logarithm of 2.</summary>
        public static T Ln2 => T.CreateChecked(Math.Log(2));

        /// <summary>ln(10), natural logarithm of 10.</summary>
        public static T Ln10 => T.CreateChecked(Math.Log(10));

        /// <summary>Degrees-to-radians conversion factor (π / 180).</summary>
        public static T Deg2Rad => T.CreateChecked(Math.PI / 180.0);

        /// <summary>Radians-to-degrees conversion factor (180 / π).</summary>
        public static T Rad2Deg => T.CreateChecked(180.0 / Math.PI);
    }

    /// <summary>
    /// Extension blocks exposing math constants as properties for common numeric types.
    /// </summary>
    public static class MathExtensions
    {
        extension(double source)
        {
            /// <inheritdoc cref="MathConstants{T}.Pi"/>
            public double Pi => MathConstants<double>.Pi;
            public double Tau => MathConstants<double>.Tau;
            public double E => MathConstants<double>.E;
            public double Phi => MathConstants<double>.Phi;
            public double Sqrt2 => MathConstants<double>.Sqrt2;
            public double Sqrt3 => MathConstants<double>.Sqrt3;
            public double Ln2 => MathConstants<double>.Ln2;
            public double Ln10 => MathConstants<double>.Ln10;
            public double Deg2Rad => MathConstants<double>.Deg2Rad;
            public double Rad2Deg => MathConstants<double>.Rad2Deg;
        }

        extension(decimal source)
        {
            public decimal Pi => MathConstants<decimal>.Pi;
            public decimal Tau => MathConstants<decimal>.Tau;
            public decimal E => MathConstants<decimal>.E;
            public decimal Phi => MathConstants<decimal>.Phi;
            public decimal Sqrt2 => MathConstants<decimal>.Sqrt2;
            public decimal Sqrt3 => MathConstants<decimal>.Sqrt3;
            public decimal Ln2 => MathConstants<decimal>.Ln2;
            public decimal Ln10 => MathConstants<decimal>.Ln10;
            public decimal Deg2Rad => MathConstants<decimal>.Deg2Rad;
            public decimal Rad2Deg => MathConstants<decimal>.Rad2Deg;
        }

        extension(float source)
        {
            public float Pi => MathConstants<float>.Pi;
            public float Tau => MathConstants<float>.Tau;
            public float E => MathConstants<float>.E;
            public float Phi => MathConstants<float>.Phi;
            public float Sqrt2 => MathConstants<float>.Sqrt2;
            public float Sqrt3 => MathConstants<float>.Sqrt3;
            public float Ln2 => MathConstants<float>.Ln2;
            public float Ln10 => MathConstants<float>.Ln10;
            public float Deg2Rad => MathConstants<float>.Deg2Rad;
            public float Rad2Deg => MathConstants<float>.Rad2Deg;
        }
    }
}

We must define extension blocks per type here.

If we move over to extension methods, we still must use a non-generic class. However, we can use for example generic math, show below. This allows to reuse code accross multiple types, supporting INumber<T> in this case.

namespace Csharp14NewFeatures
{
    using System;
    using System.Numerics;

    namespace Csharp14NewFeatures
    {
        /// <summary>
        /// Provides generic mathematical constants via extension methods for numeric types.
        /// </summary>
        public static class MathConstantExtensions
        {
            public static T GetPi<T>(this T _) where T : INumber<T> =>
                T.CreateChecked(Math.PI);

            public static T GetTau<T>(this T _) where T : INumber<T> =>
                T.CreateChecked(2 * Math.PI);

            public static T GetEuler<T>(this T _) where T : INumber<T> =>
                T.CreateChecked(Math.E);

            public static T GetPhi<T>(this T _) where T : INumber<T> =>
                T.CreateChecked((1 + Math.Sqrt(5)) / 2);

            public static T GetSqrt2<T>(this T _) where T : INumber<T> =>
                T.CreateChecked(Math.Sqrt(2));

            public static T GetSqrt3<T>(this T _) where T : INumber<T> =>
                T.CreateChecked(Math.Sqrt(3));

            public static T GetLn2<T>(this T _) where T : INumber<T> =>
                T.CreateChecked(Math.Log(2));

            public static T GetLn10<T>(this T _) where T : INumber<T> =>
                T.CreateChecked(Math.Log(10));

            public static T GetDeg2Rad<T>(this T _) where T : INumber<T> =>
                T.CreateChecked(Math.PI / 180.0);

            public static T GetRad2Deg<T>(this T _) where T : INumber<T> =>
                T.CreateChecked(180.0 / Math.PI);
        }
    }
}

Example usage of the code above :

  #region Extension metmbers using block syntax - Math

  //Extension properties 
  double radians = double.Pi / 3.0; // Pi/3 radians = 60 degrees (1 * Pi = 180 degrees) 
  double degrees = radians * radians.Rad2Deg; // Using the extension method Rad2Deg

  Console.WriteLine($"Radians: {radians:F6}"); //outputs 1.04719..
  Console.WriteLine($"Degrees: {degrees:F6}"); //outputs 60

  //Using Extension methods 

    //Using Extension methods 

  double radiansV2 = 1.0.GetPi() / 3.0;
  double degreesV2 = radians * 1.0.GetRad2Deg();

  Console.WriteLine($"Radians: {radiansV2:F6}");
  Console.WriteLine($"Degrees: {degreesV2:F6}");

Output of the code usage above:

Radians: 1,047198
Degrees: 60,000000
Radians: 1,047198
Degrees: 60,000000

So to sum up, if you use extension blocks in C#, you can use them together with generics, but the extension block must be defined to a concrete type, not using generics. This will result in some cases in lengthier code, as we cannot use generics as much as extension methods allows. Note that extension methods also must be defined inside a non-generic class.

C# 14 - Null-conditional assignments

What's new in C#

With .NET 10 released in November 2025, new features of C# is available.

Null-conditional assignment

In C# 14, Null-conditional assignment allows using the null-conditional member access operator on the left and side of assignment.

This allows more compact code, but also at the same time allow the code to become a bit more indeterministic since the code will not be run if the object on the left side of the assignment is null.

Consider this simple class :

public class AnotherClass
{
  public ShipmentService ShipmentService = new ShipmentService();
  public Order? CurrentOrder { get; set; }  
  public int? Counter { get; set; } = 0;   
}


public class ShipmentService
{
    public Order? GetCurrentOrder()
    {
        // Simulate fetching the current order, which may return null
        return null; // or return new Order { OrderId = 1234 };
    }
}

public class Order
{
    public int OrderId { get; set; }
}

We do a null check on the instance of <em>AnotherClass</em> ShipmentService here on the left side.

//Demonstrate AnotherClass using null-conditional assignment 
AnotherClass? anotherClass = null;
anotherClass?.CurrentOrder = anotherClass?.ShipmentService.GetCurrentOrder();
Console.WriteLine($"Current order retrieved using null-conditional assignment: {anotherClass?.CurrentOrder?.OrderId} Current order id is NULL? {anotherClass?.CurrentOrder?.OrderId is null}");

It is also possible to use the null check of the null-conditional assignment with compound assignment operators. The compound operators are += and -= . Note that this must be done with member access and cannot be used with values such as integers for example.

//anotherClass still NULL
anotherClass?.Counter += 2;
Console.WriteLine($"anotherClass.Counter = {anotherClass?.Counter}. Is anotherClass.Counter NULL ? {anotherClass?.Counter is null} : outputs NULL since anotherClass is still null");
anotherClass = new AnotherClass();
anotherClass?.Counter -= 15;
Console.WriteLine($"anotherClass.Counter = {anotherClass?.Counter} : outputs -15 since anotherClass is not null"); 

Output of the code above:

Current order retrieved using null-conditional assignment:  Current order id is NULL? True
anotherClass.Counter = . Is anotherClass.Counter NULL ? True : outputs NULL since anotherClass is still null
anotherClass.Counter = -15 : outputs -15 since anotherClass is not null

Testing API resilience with Polly Chaos engine

Polly is a transient failure handling and resilience library that makes it convenient to build robust APIs based on policies for handling errors that occur and offer different resilience strategies for handling these errors. The errors are not only of errors occurings either externall or internally in API, but offer also alternative strategies such as fallbacks, rate-limiting, circuit breakers and other overall act upon either reactively or proactively. A great overview of Polly can be seen in this video, although some years old now - back to 2019 - of getting an overview of Polly : NDC Oslo 2019 - System Stable: Robust connected applications with Polly, the .NET Resilience Framework - Bryan Hogan With Polly, it is possible to test out API resilience with the built in Polly Chaos engine. The Chaos engine was previously offered via the Simmy library.

Simmy - Logo

The source code in this article is available in my Github repo here: Note - the code shown in the methods below are called from Program.cs to be able to be used in the API. The sample app is an Asp.net application written with C# and with .NET 8 Target Framework. https://github.com/toreaurstadboss/HttpClientUsingPolly/

Testing out API resilience with fallbacking API endpoints

First off, the fallback strategy resilience. Polly offers a way to define fallback policies. Let's look at a way to define an HTTP client that will provide a fallback if the statuscode from the endpoint is InternalServerError = 501. The fallback is just a Json payload in this simple example.

PollyExtensions.cs

    public static void AddPollyHttpClientWithFallback(this IServiceCollection services)
    {
        services.AddHttpClient(Constants.HttpClientNames.FallbackHttpClientName, client =>
        {
            client.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("MyApp", "1.0"));
        })
        .AddResilienceHandler(
             $"{FallbackHttpClientName}{ResilienceHandlerSuffix}",
            (builder, context) =>
        {
            var serviceProvider = services.BuildServiceProvider();
            var logger = serviceProvider.GetRequiredService<ILoggerFactory>().CreateLogger(Constants.LoggerName);

            builder.AddFallback(new FallbackStrategyOptions<HttpResponseMessage>
            {
                ShouldHandle = args =>
                {
                    // Fallback skal trigges ved status 500 eller exception
                    return ValueTask.FromResult(
                        args.Outcome.Result?.StatusCode == HttpStatusCode.InternalServerError ||
                        args.Outcome.Exception is HttpRequestException
                    );
                },
                FallbackAction = args =>
                {
                    logger.LogWarning("Fallback triggered. Returning default response.");

                    var jsonObject = new
                    {
                        message = "Fallback response",
                        source = "Polly fallback",
                        timestamp = DateTime.UtcNow
                    };

                    var json = JsonSerializer.Serialize(jsonObject);

                    var fallbackResponse = new HttpResponseMessage(HttpStatusCode.OK)
                    {
                        Content = new StringContent(json, System.Text.Encoding.UTF8, "application/json")
                    };

                    return ValueTask.FromResult(Outcome.FromResult(fallbackResponse));
                }
            });

            // Inject exceptions in 80% of requests
            builder.AddChaosOutcome(new ChaosOutcomeStrategyOptions<HttpResponseMessage>()
            {
                Enabled = true,
                OutcomeGenerator = static args =>
                {
                    var response = new HttpResponseMessage(HttpStatusCode.InternalServerError);
                    return ValueTask.FromResult<Outcome<HttpResponseMessage>?>(Outcome.FromResult(response));
                },
                InjectionRate = 0.8,
                OnOutcomeInjected = args =>
                {
                    logger.LogWarning("Outcome returning internal server error");
                    return default;
                }
            });

        });
    }

Next up, let's look at the client endpoint defined with Minimal API in Aspnet core.

SampleEndpoints.cs

 app.MapGet("/test-v5-fallback", async (
 [FromServices] IHttpClientFactory httpClientFactory) =>
 {
     using var client = httpClientFactory.CreateClient(Constants.HttpClientNames.FallbackHttpClientName);

     HttpResponseMessage? response = await client.GetAsync("https://example.com");

     if (!response.IsSuccessStatusCode)
     {
         var errorContent = await response.Content.ReadAsStringAsync();
         return Results.Problem(
             detail: $"Request failed with status code {(int)response.StatusCode}: {response.ReasonPhrase}",
             statusCode: (int)response.StatusCode,
             title: "External API Error"
         );
     }

     var json = await response!.Content.ReadAsStringAsync();
     return Results.Json(json);

 });

Note the usage of [FromServices] attribute and IHttpClientFactory. The code creates the named Http client defined earlier. The fallback will return a json with fallback content in 80% of the requests in this concrete example.

Testing out API resilience with circuit breaking API endpoints

Next, the circuit breaker strategy for API resilience. Polly offers a way to define circuit breaker policies. Let's look at a way to define an HTTP client that will provide a circuit breaker if it fails for 3 consecutive requests within 30 seconds, resulting in a 10 second break. The circuit breaker strategy will stop requests that opens the circuit defined here. After the break, the circuit breaker half opens. It will accept new request, but fail immediately and open up the circuit again if it fails again, further postponing.

PollyExtensions.cs

  public static void AddPollyHttpClientWithExceptionChaosAndBreaker(this IServiceCollection services)
  {

      services.AddHttpClient(CircuitBreakerHttpClientName, client =>
      {
          client.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("MyApp", "1.0"));
      })
      .AddResilienceHandler(
          $"{CircuitBreakerHttpClientName}{ResilienceHandlerSuffix}",
          (builder, context) =>
      {
          var serviceProvider = services.BuildServiceProvider();
          var logger = serviceProvider.GetRequiredService<ILoggerFactory>().CreateLogger(Constants.LoggerName);

          //Add circuit breaker that opens after three consecutive failures and breaks for a duration of ten seconds
          builder.AddCircuitBreaker(new CircuitBreakerStrategyOptions<HttpResponseMessage>
          {
              MinimumThroughput = 3, //number of CONSECUTIVE requests failing for circuit to open (short-circuiting future requests until given BreakDuration is passed)
              FailureRatio = 1.0, //usually 1.0 is used here..
              SamplingDuration = TimeSpan.FromSeconds(30), //time window duration to look for CONSECUTIVE requests failing
              BreakDuration = TimeSpan.FromSeconds(10), //break duration. requests will be hindered at during this duration set 
              ShouldHandle = new PredicateBuilder<HttpResponseMessage>()
                  .HandleResult(r => !r.IsSuccessStatusCode)
                  .Handle<HttpRequestException>(), //defining when circuit breaker will occur given other conditions also apply
              OnOpened = args =>
              {
                  logger.LogInformation("Circuit breaker opened");
                  return default;
              },
              OnClosed = args =>
              {
                  logger.LogInformation("Circuit breaker closed");
                  return default;
              },
              OnHalfOpened = args =>
              {
                  logger.LogInformation("Circuit breaker half opened"); //half opened state happens after the circuit has been opened and break duration has passed, entering 'half-open' state (usually ONE test call must succeeed to transition from half open to open state)
                  return default;
              }
          });


          // Inject exceptions in 80% of requests
          builder.AddChaosOutcome(new ChaosOutcomeStrategyOptions<HttpResponseMessage>()
          {
              Enabled = true,
              OutcomeGenerator = static args =>
              {
                  var response = new HttpResponseMessage(HttpStatusCode.InternalServerError);
                  return ValueTask.FromResult<Outcome<HttpResponseMessage>?>(Outcome.FromResult(response));
              },
              InjectionRate = 0.8,
              OnOutcomeInjected = args =>
              {
                  logger.LogWarning("Outcome returning internal server error");
                  return default;
              }
          });


      });
  }

Let's look at the client endpoint defined with Minimal API in Aspnet core for circuit-breaker example.

SampleEndpoints.cs

  app.MapGet("/test-v4-circuitbreaker-opening", async (
  [FromServices] IHttpClientFactory httpClientFactory) =>
  {
      using var client = httpClientFactory.CreateClient(Constants.HttpClientNames.CircuitBreakerHttpClientName);

      HttpResponseMessage? response = await client.GetAsync("https://example.com");

      if (!response.IsSuccessStatusCode)
      {
          var errorContent = await response.Content.ReadAsStringAsync();
          return Results.Problem(
              detail: $"Request failed with status code {(int)response.StatusCode}: {response.ReasonPhrase}",
              statusCode: (int)response.StatusCode,
              title: "External API Error"
          );
      }

      var json = await response!.Content.ReadAsStringAsync();
      return Results.Json(json);

  });

Testing out API resilience for latency induced timeout API endpoints

Next, the timeout strategy for API resilience. Polly offers a way to define timeout policies and can combine these for testing by injecting latency (additional execution time). Let's look at a way to define an HTTP client that will provide a timeout if it times out already after one second with a 50% chance of getting a 3 second latency, which will trigger the timeout.

PollyExtensions.cs

   public static void AddPollyHttpClientWithIntendedRetriesAndLatencyAndTimeout(this IServiceCollection services)
  {
      services.AddHttpClient(RetryingTimeoutLatencyHttpClientName, client =>
      {
          client.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("MyApp", "1.0"));
      })
      .AddResilienceHandler(
          $"{RetryingTimeoutLatencyHttpClientName}{ResilienceHandlerSuffix}",
      (builder, context) =>
       {
           var serviceProvider = services.BuildServiceProvider();
           var logger = serviceProvider.GetRequiredService<ILoggerFactory>().CreateLogger(Constants.LoggerName);

           // Timeout strategy : fail if request takes longer than 1s
           builder.AddTimeout(new HttpTimeoutStrategyOptions
           {
               Timeout = TimeSpan.FromSeconds(1),
               OnTimeout = args =>
               {
                   logger.LogWarning($"Timeout after {args.Timeout.TotalSeconds} seconds");
                   return default;
               }
           });

           // Chaos latency: inject 3s delay in 30% of cases
           builder.AddChaosLatency(new ChaosLatencyStrategyOptions
           {
               InjectionRate = 0.5,
               Latency = TimeSpan.FromSeconds(3),
               Enabled = true,
               OnLatencyInjected = args =>
               {
                   logger.LogInformation("... Injecting a latency of 3 seconds ...");
                   return default;
               }
           });

           // Chaos strategy: inject 500 Internal Server Error in 75% of cases
           builder.AddChaosOutcome<HttpResponseMessage>(
               new ChaosOutcomeStrategyOptions<HttpResponseMessage>
               {
                   InjectionRate = 0.5,
                   Enabled = true,
                   OutcomeGenerator = static args =>
                   {
                       var response = new HttpResponseMessage(HttpStatusCode.InternalServerError);
                       return ValueTask.FromResult<Outcome<HttpResponseMessage>?>(Outcome.FromResult(response));
                   },
                   OnOutcomeInjected = args =>
                   {
                       logger.LogWarning("Outcome returning internal server error");
                       return default;
                   }
               });
       });

  }

Let's look at the client endpoint defined with Minimal API in Aspnet core for timeout with latency example.

SampleEndpoints.cs

 app.MapGet("/test-v3-latency-timeout", async (
 [FromServices] IHttpClientFactory httpClientFactory) =>
 {
     using var client = httpClientFactory.CreateClient(Constants.HttpClientNames.RetryingTimeoutLatencyHttpClientName);

     var response = await client.GetAsync("https://example.com");

     if (!response.IsSuccessStatusCode)
     {
         var errorContent = await response.Content.ReadAsStringAsync();
         return Results.Problem(
             detail: $"Request failed with status code {(int)response.StatusCode}: {response.ReasonPhrase}",
             statusCode: (int)response.StatusCode,
             title: "External API Error"
         );
     }

     var json = await response.Content.ReadAsStringAsync();
     return Results.Json(json);

 });

The following screenshot shows the timeout occuring after the defined setup of induced latency by given probability and defined timeout.

Testing out API resilience with retries

Retries offers an API endpoint to gain more robustness, by allowing multiple retries and define a strategy for these retries.The example http client here also adds a chaos outcome internal server error = 501 that is thrown with 75% probability (failure rate).

PollyExtensions.cs

    public static void AddPollyHttpClientWithIntendedRetries(this IServiceCollection services)
   {
       services.AddHttpClient(RetryingHttpClientName, client =>
           {
               client.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("MyApp", "1.0"));
           })
           .AddResilienceHandler("polly-chaos", (builder, context) =>
           {
               var serviceProvider = services.BuildServiceProvider();
               var logger = serviceProvider.GetRequiredService<ILoggerFactory>().CreateLogger(Constants.LoggerName);

               //Retry strategy
               builder.AddRetry(new RetryStrategyOptions<HttpResponseMessage>
               {
                   ShouldHandle = new PredicateBuilder<HttpResponseMessage>()
                       .HandleResult(r => !r.IsSuccessStatusCode)
                       .Handle<HttpRequestException>(),
                   MaxRetryAttempts = 3,
                   DelayGenerator = RetryDelaysPipeline,
                   OnRetry = args =>
                   {
                       logger.LogWarning($"Retrying {args.AttemptNumber} for requesturi {args.Context.GetRequestMessage()?.RequestUri}");
                       return default;
                   }
               });

               // Chaos strategy: inject 500 Internal Server Error in 75% of cases
               builder.AddChaosOutcome<HttpResponseMessage>(
                   new ChaosOutcomeStrategyOptions<HttpResponseMessage>
                   {
                       InjectionRate = 0.75,
                       Enabled = true,
                       OutcomeGenerator = static args =>
                       {
                           var response = new HttpResponseMessage(HttpStatusCode.InternalServerError);
                           return ValueTask.FromResult<Outcome<HttpResponseMessage>?>(Outcome.FromResult(response));
                       }
                   });

           });

   }

Let's look at the client endpoint defined with Minimal API in Aspnet core for the retrying example.

SampleEndpoints.cs

   app.MapGet("/test-retry-v2", async (
       [FromServices] IHttpClientFactory httpClientFactory) =>
   {
       using var client = httpClientFactory.CreateClient(Constants.HttpClientNames.RetryingHttpClientName);

       var response = await client.GetAsync("https://example.com");

       return Results.Json(response);
   });

There are multiple resilience scenarios that Polly offers, the table below lists them up (this article has presented most of them):

Summary

The following summary explains what this article has presented.

🧪 Testing API Resilience with Polly Chaos Engineering In this article, we have explored how to build resilient APIs using Polly v9 and its integrated chaos engine. Through practical examples, the article has demonstrated how to simulate real-world failures—like latency, timeouts, and internal server errors—and apply resilience strategies such as fallbacks, retries, and circuit breakers. By injecting controlled chaos, developers can proactively test and strengthen their systems against instability and external dependencies. Polly v9 library offers additionally scenarios also for making robust APIs. Info: Asp net core is used in this article, together with C# and .NET 8 as Target Framework.

Azure Keyvault and HMAC-SHA256 Pseudonymizer

In this article, using Azure Keyvault together with HMAC-SHA256.

Introduction to HMAC SHA-256

The HMAC-SHA256 is a SHA-256 cryptographic hashing function that combines SHA-256 with a secret key (that can be shared) to provide both data integrity (to ensure that data has not been altered) and data authenticity (that data originates from a sender that knows about the shared secret key of the HMAC-SHA256 hash). HMAC-SHA256 generates a Message Authentication Code (MAC)
that we can get both these benefits from, data authenticity and data integrity. Since it is 256 bits or 32 bytes, the hash can be represented as hexadecimal string of length 64. If it is represented as a base-64 string, it will be approximately 44 bytes long, given that a base-64 string will be padded in multiple of 4 characters.

Storing the shared secret key in Azure Keyvault

Azure Keyvault will be used to store the shared secret key. The secret key could be stored anywhere, but using a KeyVault in Azure makes it easier to keep it safe and apply rolling policies for the secret key. First off, to create an Azure Keyvault secret, make sure you have an Azure Keyvault created. Search for a resource called Key vaults with a key icon. Choose Create, enter a Key vault name and Region and resource group, Pay per use and Standard pricing tier. (Premium includes HSM backed keys, which will use dedicated physical devices - HSM = Hardware Security Module - for enhanced FIPS 140-2 Level 3 certified security). Once the keyvault is created, choose Object and Secrets. Click Generate/Import. Just enter a name for the secret and the secret value. It is suggested that they secret value you enter should be genereated from a cryptographically secure random generator and a length of at least 16 bytes and 32 bytes.

Generating a strong shared secret key for HMAC SHA-256

The following code can be used to generate a strong 256 (32-byte) key, and pasted into the Secret value

HmacSha256KeyGenerator.cs

using System.Security.Cryptography;

namespace MinimalApiSecurityExperimentsDotNet.Extensions
{

    public static class HmacSha256KeyGenerator
    {

        public static string GenerateStrongKey()
        {
            // Generate a 256-bit (32-byte) key
            byte[] key = new byte[32];
            using (var rng = RandomNumberGenerator.Create())
            {
                rng.GetBytes(key);
            }

            // Convert to Base64 for storage or display
            string base64Key = Convert.ToBase64String(key);
            return base64Key;
            //Console.WriteLine("Generated HMAC-SHA-256 Key (Base64):");
            //Console.WriteLine(base64Key);
        }

    }

}

MinimalApiSecurityDemo.csproj

The following Nuget packages can be added to work with Azure Keyvault secrets.

  <PackageReference Include="Azure.Identity" Version="1.14.2" />
  <PackageReference Include="Azure.Security.KeyVault.Keys" Version="4.8.0" />
  <PackageReference Include="Azure.Security.KeyVault.Secrets" Version="4.8.0" />

Note that you do not need the Keys nuget package here if you only want to retrieve secrets. Azure KeyVault got three different object types - Keys, secrets and certificates. Only secrets will be looked at in this article.

KeyVaultSecretRetriever.cs

Let's look at key vault retrieval.

using Azure.Identity;
using Azure.Security.KeyVault.Secrets;

namespace MinimalApiSecurityExperimentsDotNet
{
    
    public class KeyVaultSecretRetriever
    {
        private readonly IConfiguration _configuration;
        public KeyVaultSecretRetriever(IConfiguration configuration)
        {
            _configuration = configuration;
        }

        public async Task<KeyVaultSecret>? GetSecretAsync(string secretName)
        {
            try
            {
                string keyVaultUri = _configuration["AzureKeyVault:VaultUri"] ?? throw new ArgumentNullException(nameof(keyVaultUri));
                var client = new SecretClient(new Uri(keyVaultUri), new DefaultAzureCredential());
                KeyVaultSecret secret = await client.GetSecretAsync(secretName);
                return secret;
            }
            catch (Exception ex)
            {
                Console.WriteLine($"Error retrieving the secret {secretName}: {ex.Message}");
                return null;
            }
        }     

    }

}

The SecretClient in the code above is the client used to retrieve the secret from Azure KeyVault. The client is from the Azure.Security.KeyVault.Secrets Nuget package. The DefaultAzureCredential used here means you will have to login in advance before retrieving the secret. There are different credential types you can use instead, but this default credential type is the easiest to use for testing out. If you run the code above from the console, you can for example install the Azure Powershell module and run az login. The following appsettings file can be used for the code above.

appsettings.json

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*",

  "AzureKeyVault": {
    "VaultUri": "https://yourkeyvaultsomewhereinazure.vault.azure.net/",
    "HmacSha256SecretName":  "YourExampleHmacStrongKey"
  }

}

Pseudonymizer class

The following pseudonymizer class uses HMAC-SHA256 to generate a pseudonym for a given message. This can be used for generating a value for sensitive data with a key. The key can be shared between sender and receiver to avoid exposing sensitive values in clear text.

Pseudonymizer.cs

using System.Text;

namespace MinimalApiSecurityExperimentsDotNet
{

    public class Pseudonymizer
    {
        private readonly byte[]? _key;

        public Pseudonymizer(string key)
        {
            if (!string.IsNullOrWhiteSpace(key))
            {
                _key = Encoding.UTF8.GetBytes(key);
            }
        }

        /// <summary>
        /// Outputs a pseudonymized version of the input string using HMAC SHA256.       
        /// </summary>
        /// <param name="message"></param>
        /// <param name="outputToHexString">Output to hex string if true. If false, base-64 string is returned</param>
        /// <returns></returns>
        public string? Pseudonymize(string? message, bool outputToHexString = true)
        {
            if (string.IsNullOrWhiteSpace(message) || _key == null)
            {
                return message;
            }
            using var hmac = new System.Security.Cryptography.HMACSHA256(_key);
            var hash = hmac.ComputeHash(Encoding.UTF8.GetBytes(message));
            return outputToHexString ? Convert.ToHexString(hash) : Convert.ToBase64String(hash);
        }

        public bool Verify(string input, string pseudonym)
        {
            if (string.IsNullOrEmpty(input) || string.IsNullOrEmpty(pseudonym))
            {
                return false;
            }
            var computedPseudonym = Pseudonymize(input);
            return computedPseudonym == pseudonym;
        }

    }

}

The code above shows that we can both psedonymize and verify the pseudonym. Next up, a provider for creating an instance of this Pseudonymizer for use in ASP.net Core, for example.

PseudonymizerProvider.cs

namespace MinimalApiSecurityExperimentsDotNet.Extensions
{

    public class PseudonymizerProvider
    {

        private Pseudonymizer? _pseudonymizer;

        public void Set(Pseudonymizer pseudonymizer)
        {
            _pseudonymizer = pseudonymizer;
        }

        public Pseudonymizer Get()
        {
            return _pseudonymizer ?? throw new InvalidOperationException("Pseudonymizer not initalized");
        }

    }

}

The following Hosted service sets up the pseudonymizer service. It will start up when the Asp.net core application is starting, however after the DI container has been built.

PseudonymizerHostedService .cs

using Azure.Security.KeyVault.Secrets;

namespace MinimalApiSecurityExperimentsDotNet.Extensions
{

    public class PseudonymizerHostedService : IHostedService
    {

        private readonly IServiceProvider _provider;

        public PseudonymizerHostedService(IServiceProvider provider)
        {
            _provider = provider;
        }

        public async Task StartAsync(CancellationToken cancellationToken)
        {
            using var scope = _provider.CreateScope();
            var config = scope.ServiceProvider.GetRequiredService<IConfiguration>();
            var keyVaultRetriever = new KeyVaultSecretRetriever(config);
            var hmacSha256SecretName = config["AzureKeyVault:HmacSha256SecretName"];
            if (hmacSha256SecretName == null)
            {
                throw new InvalidOperationException($"Config '{hmacSha256SecretName}' not is missing.");
            }
            KeyVaultSecret? secret = await keyVaultRetriever.GetSecretAsync(hmacSha256SecretName);

            if (secret == null)
            {
                throw new InvalidOperationException($"Secret '{hmacSha256SecretName}' not found in Key Vault.");
            }

            var pseudonymizer = new Pseudonymizer(secret.Value);

            // Register the pseudonymizer in the pseudonymizerProvider
            var pseudonymizerProvider = scope.ServiceProvider.GetRequiredService<PseudonymizerProvider>();
            pseudonymizerProvider.Set(pseudonymizer);
        }

        public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;

    }

}

In the Program.cs of a sample Asp.net Web API with minimal API, the provider and the hosted service is registered before building the webapplication with .Build().

Program.cs

using MinimalApiSecurityExperimentsDotNet.Extensions;

namespace MinimalApiSecurityExperimentsDotNet
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var builder = WebApplication.CreateBuilder(args);

            // Add services to the container.

            builder.Services.AddControllers();

            // Add Swagger services
            builder.Services.AddSwaggerGen();
            builder.Services.AddEndpointsApiExplorer();

            // Adding keyvault access via registering PseudonymizerHostedService 
            builder.Services.AddSingleton<PseudonymizerProvider>();
            builder.Services.AddHostedService<PseudonymizerHostedService>();

            var app = builder.Build();

            // Configure the HTTP request pipeline.

            app.UseHttpsRedirection();

            app.UseAuthorization();

            app.MapControllers();

            // Enable Swagger middleware 
            if (app.Environment.IsDevelopment())
            {
                app.UseSwagger();
                app.UseSwaggerUI();
            }
          
            app.MapGet("/generatestrongkey", () =>
            {
                return HmacSha256KeyGenerator.GenerateStrongKey();

            });

            app.MapGet("/pseudonymize", (string? key, string? value) =>
            {
                var pseudonym = new Pseudonymizer(key).Pseudonymize(value, outputToHexString: true);
                return Results.Json(pseudonym);
            });

            app.MapGet("/pseudonymize/verify", (string? key, string? value, string? pseudonym) =>
            {
                var pseudonymizer = new Pseudonymizer(key);
                var isValid = pseudonymizer.Verify(value, pseudonym);
                return Results.Json(isValid);
            });

            //Example using the injected singleton PseudonymizerProvider

            app.MapGet("/pseudo", (string? input, PseudonymizerProvider pseudonymizerProvider) =>
            {
                var pseudonymizer = pseudonymizerProvider.Get();
                var result = pseudonymizer.Pseudonymize(input); //pseudonymize the input using the inject Pseudonymizer instance. This will use HMAC SHA256 under the hood.
                return Results.Ok(new { message = input, pseudonym = result });
            });

            app.MapGet("/getkeyvaultsecret", async (string secretName) =>
            {
                var keyvaultRetriever = new KeyVaultSecretRetriever(builder.Configuration);
                var secret = await keyvaultRetriever.GetSecretAsync(secretName);
                return Results.Json(secret);
            });          

            app.Run(); 
        }
    }


}

The benefits of registering the pseudonymizer's hosted service and provider in the container is shown in the code above. We can easily pseudonymize and use Azure Key vault stored secret for the HMAC-SHA256 hash / pseudonoym.

 //Example using the injected singleton PseudonymizerProvider

app.MapGet("/pseudo", (string? input, PseudonymizerProvider pseudonymizerProvider) =>
{
    var pseudonymizer = pseudonymizerProvider.Get();
    var result = pseudonymizer.Pseudonymize(input); //pseudonymize the input using the inject Pseudonymizer instance. This will use HMAC SHA256 under the hood.
    return Results.Ok(new { message = input, pseudonym = result });
});

Summary

The pseudonymizer is initialized and registered as a singleton, it got a dependency against Azure Keyvault service at startup, but further on pseudonymizing can now use the singleton instance of the pseudonymizer that is registered by the hosted service at application startup. The pseudonymizer can for example pseudonymize sensitive values and hide their content. A receiver can be shown this pseudonymized value in the response. There pseudonymized value could also be described as for example one of these three descriptions:

• Key identifier / alias
• Obfuscated key named
• Derived key label

If the receiver got the shared secret key, since it is a one-way cryptographic algorithm, obtaining the original value is still only possible via a brute-force dictionary attack. However, it can be used for example as a pseudo-id that is shared and also identify for example a shared identifier, for example a patient identifier for multiple registered documents in a database for example. The pseudo id can be even stored in the database as a way of obfuscating sensitive values such a SSN / PID (Patient Id, SSN = Social Security Number).

Note about Keyvault access

Just because you know the url to an Azure keyvault and possible the secret's name, does of course not mean you got access to the secret. Inside the Azure Keyvault, clik on Access Control (IAM) to get started. To just test things out, follow the guide by clicking the button Add role assignment. Follow the wizard to gain access to your key vault. Search for role Key Vault Administrator. Click Next. Choose Select members to choose for example your own user. Click Next Click Review and assign. Finally you have added access to your Key vault. This must be done even for the Key vault you have created yourself. Of course, if you use Azure AD, you can choose different members here and authorize fine-grained access.

Blazorise Datepicker with highlighting of dates

The Blazorise Datepicker is a date picker control for setting dates in Blazor WASM web assembly apps. It uses Flatpickr to provide a user friendly UI for selecting a date and navigate among months and years. It is used in web apps that use Blazor. The date picker is documented here :
https://blazorise.com/docs/components/date-picker
This date picker does not support highlighting certain dates. It do support enabling specified dates or disabling specified dates. But highlighting dates is not possible. I have added a Github repo where this is added support for :

https://github.com/toreaurstadboss/BlazoriseDatePickerWithHolidays

First off, the following custom Blazor component provides the custom date picker with the support for highlighting specified dates.

CustomDatePicker.razor

@using Blazorise
@using Blazorise.Localization
@using BlazoriseDatePickerWithHolidays.Service
@using System.Globalization
@using static BlazoriseDatePickerWithHolidays.Service.HolidayService
@inject IJSRuntime JS
@inject ITextLocalizerService TextLocalizerService

<Addons>
    <Addon AddonType="AddonType.Body">
        <DatePicker TValue="DateTime"
                    Date="@SelectedDate"
                    FirstDayOfWeek="@FirstDayOfWeek"
                    DateChanged="@OnDateChanged"
                    InputFormat="@InputFormat"   
                    InputMode="@InputMode"
                    DisplayFormat="@DisplayFormat"                    
                    Placeholder="@Placeholder"
                    TimeAs24hr="@TimeAs24hr"
                    @attributes="@AdditionalAttributes"
                    @ref="datePicker" />
    </Addon>
    <Addon AddonType="AddonType.End">
        <Button Class="addon-margin" Color="Color.Primary" Clicked="@(() => datePicker?.ToggleAsync())">
            <Icon Name="IconName.CalendarDay" />
        </Button>
    </Addon>
</Addons>

@code {
    /// <summary>
    /// List of dates to be highlighted in the date picker. Each date can have an annotation (e.g., holiday name) that will be shown as a tooltip.
    /// </summary>
    [Parameter]
    [EditorRequired]
    public List<AnnotatedDateTime> HighlightedDays { get; set; }

    /// <summary>
    /// The CSS class to apply to highlighted dates in the calendar.
    /// </summary>
    [Parameter]
    public string HighlightCssClass { get; set; } = "pink-day"; //Default CSS class pink-day is a custom CSS class defined in wwwroot/css/app.css

    /// <summary>
    /// The first day of the week in the date picker calendar. Defaults to Monday.
    /// </summary>
    [Parameter]
    public DayOfWeek FirstDayOfWeek { get; set; } = DayOfWeek.Monday;

    /// <summary>
    /// Whether to display time in 24-hour format. Defaults to true.
    /// </summary>
    [Parameter]
    public bool TimeAs24hr { get; set; } = true;

    /// <summary>   
    /// The locale to use for the date picker, which affects date formatting and localization. 
    /// Supported locales : https://blazorise.com/docs/helpers/localization
    /// </summary>
    [Parameter]
    public string Locale { get; set; } = "en-US";

    /// <summary>
    /// The input mode for the date picker, which can be either Date or DateTime or Month.
    /// </summary>
    [Parameter]
    public DateInputMode InputMode { get; set; } = DateInputMode.Date;

    /// <summary>
    /// The currently selected date in the date picker.
    /// </summary>
    [Parameter] 
    public DateTime SelectedDate { get; set; }

    /// <summary>
    /// Event callback triggered when the selected date changes.
    /// </summary>
    [Parameter] 
    public EventCallback<DateTime> SelectedDateChanged { get; set; }

    /// <summary>
    /// The input format string for displaying the date in the input field.
    /// </summary>
    [Parameter] 
    public string? InputFormat { get; set; } = "dd.MM.yyyy";

    /// <summary>
    /// The display format string for showing the date in the calendar.
    /// </summary>
    [Parameter] 
    public string? DisplayFormat { get; set; } = "dd.MM.yyyy";

    /// <summary>
    /// Placeholder text to display when no date is selected.
    /// </summary>
    [Parameter] 
    public string? Placeholder { get; set; }

    /// <summary>
    /// Whether to show the clear button in the date picker.
    /// </summary>
    [Parameter] 
    public bool ShowClearButton { get; set; } = true;

    /// <summary>
    /// Additional attributes to be splatted onto the underlying DatePicker component.
    /// </summary>
    [Parameter(CaptureUnmatchedValues = true)] 
    public Dictionary<string, object>? AdditionalAttributes { get; set; }

    /// <summary>
    /// Handles component rendering and registers JS interop for month and year changes.
    /// </summary>
    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            _dotNetRef = DotNetObjectReference.Create(this);
            await JS.InvokeVoidAsync("registerFlatpickrMonthChange", "input.flatpickr-input", _dotNetRef);
            await JS.InvokeVoidAsync("registerFlatpickrYearChange", "input.flatpickr-input", _dotNetRef);
        }
        await ReloadDatesToHighlight();
    }

    protected override void OnParametersSet()
    {
        if (Locale != null){
            TextLocalizerService.ChangeLanguage(Locale);
        }
    }

    /// <summary>
    /// Invoked from JS when the month is changed in the calendar.
    /// </summary>
    [JSInvokable]
    public async Task OnMonthChanged()
    {
        await ReloadDatesToHighlight();
    }

    /// <summary>
    /// Invoked from JS when the year is changed in the calendar.
    /// </summary>
    [JSInvokable]
    public async Task OnYearChanged()
    {
        await ReloadDatesToHighlight();
    }

    /// <summary>
    /// Disposes JS interop references and the DatePicker component.
    /// </summary>
    public async ValueTask DisposeAsync()
    {
        _dotNetRef?.Dispose();
        if (datePicker != null)
            await datePicker.DisposeAsync();
    }
    
    /// <summary>
    /// Highlights the specified dates in the calendar using JS interop.
    /// </summary>
    private async Task ReloadDatesToHighlight()
    {
        if (HighlightedDays == null)
            return;

        string chosenLocale = this.Locale ?? "en-US";

        //Console.WriteLine("chosenLocale:" + chosenLocale);

        var datesToHighlight = HighlightedDays
            .Select(d => new { annotation = d.Annotation, date = d.Value.ToString("MMMM d, yyyy", new CultureInfo(chosenLocale)) })
            .ToArray();

        //Console.WriteLine(System.Text.Json.JsonSerializer.Serialize(datesToHighlight));

        await JS.InvokeVoidAsync(
            "highlightFlatpickrDates",
            ".flatpickr-calendar",
            datesToHighlight,
            HighlightCssClass // Pass the class as an extra argument
        );
    }

    /// <summary>
    /// Optionally provides a CSS class for a given date. Not used in this implementation.
    /// </summary>
    private string GetDateClass(DateTime date)
    {
        return string.Empty;
    }

    /// <summary>
    /// Handles the date change event from the DatePicker and propagates it to the parent component.
    /// </summary>
    private async Task OnDateChanged(DateTime newValue)
    {
        SelectedDate = newValue;
        await SelectedDateChanged.InvokeAsync(newValue);
    }

    private DatePicker<DateTime>? datePicker;
    private DotNetObjectReference<CustomDatePicker>? _dotNetRef;
}

Notice the trick done to capture unmatched values in case some parameters of DatePicker in the custom date picker is missing and we still want the user of this component to set a parameter of the date picker component :

    /// 

    /// Additional attributes to be splatted onto the underlying DatePicker component.
    /// 
    [Parameter(CaptureUnmatchedValues = true)] 
    public Dictionary<string, object>? AdditionalAttributes { get; set; }

The calls to IJSRunTime to register callback script handlers will register callbacks for the component, this is done in this line :

_dotNetRef = DotNetObjectReference.Create(this);
await JS.InvokeVoidAsync("registerFlatpickrMonthChange", "input.flatpickr-input", _dotNetRef); 
await JS.InvokeVoidAsync("registerFlatpickrYearChange", "input.flatpickr-input", _dotNetRef);           

The following client side script is registeret into the global window object to provide script event handlers that will be called when the end-user changes selected year or month, either via the UI controls or just entering the date.

datepicker-highlight.js

window.highlightFlatpickrDates = (selector, dates, highlightCssClass) => {
    //debugger
    const calendar = document.querySelector(selector); 
    if (!calendar) {
        return; //Wait for Flatpickr to render days
    }
    //debugger
    setTimeout(() => {
        //
        dates.forEach(date => {
            //debugger
            const dayElem = calendar.querySelector(`.flatpickr-day[aria-label="${date.date}"]`);
            if (dayElem) {
                dayElem.classList.add(highlightCssClass);
                dayElem.setAttribute('title', date.annotation);
            }
        });
    }, 50);
};

window.registerFlatpickrMonthChange = (selector, dotNetHelper) => {
    const fpInput = document.querySelector(selector);
    if (!fpInput || !fpInput._flatpickr) {
        setTimeout(() => window.registerFlatpickrMonthChange(selector, dotNetHelper), 50);
        return;
    }
    fpInput._flatpickr.config.onMonthChange.push(function () {
        dotNetHelper.invokeMethodAsync('OnMonthChanged');
    });
};

window.registerFlatpickrYearChange = (selector, dotNetHelper) => {
    const fpInput = document.querySelector(selector);
    if (!fpInput || !fpInput._flatpickr) {
        setTimeout(() => window.registerFlatpickrYearChange(selector, dotNetHelper), 50);
        return;
    }
    fpInput._flatpickr.config.onYearChange.push(function () {
        dotNetHelper.invokeMethodAsync('OnYearChanged');
    });
};

The following test page test out the component.

StyledDatePicker.razor

@page "/"
@using BlazoriseDatePickerWithHolidays.Components
@using BlazoriseDatePickerWithHolidays.Service
@using System.Globalization
@using static BlazoriseDatePickerWithHolidays.Service.HolidayService
@inject IHolidayService HolidayService

<h3>Custom Date Picker with Highlighted Days</h3>

<p>

    <b>The date picker used here is from Blazorise library and customized to allow setting days to highlight including tooltips annotating a description of the date being highlighted.</b>
    <br />
    Blazorise DatePicker component is described here:
    <br /><br />
    <a href="https://blazorise.com/docs/components/date-picker">https://blazorise.com/docs/components/date-picker</a>
    <br /><br />
    The highlighted dates selected for this demo are Christian Holidays and other public days off in Norway.
    In Norwegian, these days off are called 'Offentlige høytidsdager'.<br />
    These days are marked with a pink background, white foreground and rounded corners. <br />
    Tooltips are added showing the Holiday name <br/>

</p>

<div class="container-fluid">
    <div class="row">
        <div class="col-md-2">
            <CustomDatePicker
                @bind-SelectedDate="selectedDate" 
                Locale="en-US"
                FirstDayOfWeek="DayOfWeek.Monday"
                InputMode="DateInputMode.DateTime"
                DisplayFormat="HH:mm dd.MM.yyyy"
                InputFormat="HH:mm dd.MM.yyyy"
                TimeAs24hr="true"
                HighlightedDays="_holidays"
                @ref="@_datePicker" />
        </div>
    </div>
</div>

@{
    var chosenLocale = new CultureInfo(_datePicker?.Locale ?? "en-US");
}

<p class="mt-3">Selected date <strong>@selectedDate.ToString("HH:mm dd.MM.yyyy", chosenLocale)</strong></p>

@code {
    private DateTime selectedDate = new DateTime(2025, 5, 4);
    private static int minYear = DateTime.Today.AddYears(-20).Year;
    private List<AnnotatedDateTime>? _holidays;

    private CustomDatePicker? _datePicker;

    protected override void OnParametersSet()
    {  
        _holidays = Enumerable.Range(minYear, 40)
            .SelectMany(y => HolidayService.GetHolidays(y))
            .ToList();
    }

}

The dates to select uses an implementation of IHolidayService. This implemented like this, note that this finds 'Offentlige høytidsdager' and (Christian-belief) holidays in Norway.

HolidayService.cs

namespace BlazoriseDatePickerWithHolidays.Service;

public interface IHolidayService
{
    DateTime AddWorkingDaysToDate(DateTime date, int days);
    List<string> GetHolidayNames();
    IEnumerable<HolidayService.AnnotatedDateTime> GetHolidays(int year);
    bool IsHoliday(DateTime date);
    bool IsWorkingDay(DateTime date);
}

public partial class HolidayService : IHolidayService
{

    private static MemoryCache<int, HashSet<AnnotatedDateTime>> holidays = new();

    private static readonly List<string> holidayNames = new List<string> {
            "1. nyttårsdag",
            "Palmesøndag",
            "Skjærtorsdag",
            "Langfredag",
            "1. påskedag",
            "2. påskedag",
            "Offentlig høytidsdag",
            "Grunnlovsdag",
            "Kristi Himmelfartsdag",
            "1. pinsedag",
            "2. pinsedag",
            "1. juledag",
            "2. juledag"
    };

    /// <summary>
    /// Adds the given number of working days to the given date. A working day is
    /// specified as a regular Norwegian working day, excluding weekends and all
    /// national holidays.
    /// 
    /// Example 1:
    /// - Add 5 working days to Wednesday 21.03.2007 -> Yields Wednesday
    /// 28.03.2007. (skipping saturday and sunday)
    /// 
    /// Example 2:
    /// - Add 5 working days to Wednesday 04.04.2007 (day before
    /// easter-long-weekend) -> yields Monday 16.04.2007 (skipping 2 weekends and
    /// 3 weekday holidays).
    /// </summary>
    /// <param name="date">The original date</param>
    /// <param name="days">The number of working days to add</param>
    /// <returns>The new date</returns>
    public DateTime AddWorkingDaysToDate(DateTime date, int days)
    {
        var localDate = date;
        for (var i = 0; i < days; i++)
        {
            localDate = localDate.AddDays(1);
            while (!IsWorkingDay(localDate))
            {
                localDate = localDate.AddDays(1);
            }
        }
        return localDate;
    }

    /// <summary>
    /// Will check if the given date is a working day. That is check if the given
    /// date is a weekend day or a national holiday.
    /// </summary>
    /// <param name="date">The date to check</param>
    /// <returns>true if the given date is a working day, false otherwise</returns>
    public bool IsWorkingDay(DateTime date)
    {
        return date.DayOfWeek != DayOfWeek.Saturday && date.DayOfWeek != DayOfWeek.Sunday
               && !IsHoliday(date);
    }

    public List<string> GetHolidayNames()
    {
        return holidayNames;
    }

    /// <summary>
    /// Check if given Date object is a holiday.
    /// </summary>
    /// <param name="date">date to check if is a holiday</param>
    /// <returns>true if holiday, false otherwise</returns>
    public bool IsHoliday(DateTime date)
    {
        var year = date.Year;
        var holidaysForYear = GetHolidaySet(year);
        foreach (var holiday in holidaysForYear)
        {
            if (CheckDate(date, holiday.Value))
            {
                return true;
            }
        }
        return false;
    }

    /// <summary>
    /// Return a sorted array of holidays for a given year.
    /// </summary>
    /// <param name="year">The year to get holidays for</param>
    /// <returns>Holidays, sorted by date</returns>
    public IEnumerable<AnnotatedDateTime> GetHolidays(int year)
    {
        var days = GetHolidaySet(year);
        var listOfHolidays = new List<AnnotatedDateTime>(days);
        listOfHolidays.Sort((date1, date2) => date1.Value.CompareTo(date2.Value));
        return listOfHolidays;
    }

    /// <summary>
    /// Get a set of holidays for a given year
    /// </summary>
    /// <param name="year">The year to get holidays for</param>
    /// <returns>Holidays for year</returns>
    private IEnumerable<AnnotatedDateTime> GetHolidaySet(int year)
    {
        if (holidays == null)
        {
            holidays = new MemoryCache<int, HashSet<AnnotatedDateTime>>();
        }
        if (holidays.Get(year) == null)
        {
            var yearSet = new HashSet<AnnotatedDateTime>();

            // Add set holidays.
            yearSet.Add(new AnnotatedDateTime("1. nyttårsdag " + year, new DateTime(year, 1, 1)));
            yearSet.Add(new AnnotatedDateTime("Offentlig høytidsdag " + year, new DateTime(year, 5, 1)));
            yearSet.Add(new AnnotatedDateTime("Grunnlovsdag " + year, new DateTime(year, 5, 17)));
            yearSet.Add(new AnnotatedDateTime("1. juledag " + year, new DateTime(year, 12, 25)));
            yearSet.Add(new AnnotatedDateTime("2. juledag " + year, new DateTime(year, 12, 26)));

            // Add movable holidays - based on easter day.
            var easterDay = GetEasterDay(year);

            // Sunday before easter.
            yearSet.Add(new AnnotatedDateTime("Palmesøndag " + year, easterDay.AddDays(-7)));

            // Thurday before easter.
            yearSet.Add(new AnnotatedDateTime("Skjærtorsdag " + year, easterDay.AddDays(-3)));

            // Friday before easter.
            yearSet.Add(new AnnotatedDateTime("Langfredag " + year, easterDay.AddDays(-2)));

            // Easter day.
            yearSet.Add(new AnnotatedDateTime("1. påskedag " + year, easterDay));

            // Second easter day.
            yearSet.Add(new AnnotatedDateTime("2. påskedag " + year, easterDay.AddDays(1)));

            // "Kristi himmelfart" day.
            yearSet.Add(new AnnotatedDateTime("Kristi Himmelfartsdag " + year, easterDay.AddDays(39)));

            // "Pinse" day.
            yearSet.Add(new AnnotatedDateTime("1. pinsedag " + year, easterDay.AddDays(49)));

            // Second "Pinse" day.
            yearSet.Add(new AnnotatedDateTime("2. pinsedag " + year, easterDay.AddDays(50)));

            holidays.Add(year, yearSet);
        }
        return holidays.GetAsValue(year)!;
    }

    /// <summary>
    ///  Calculates easter day (sunday) by using Spencer Jones formula found here:
    ///  http://no.wikipedia.org/wiki/P%C3%A5skeformelen
    /// </summary>
    /// <param name="year">year</param>
    /// <returns>easterday for year</returns>
    private DateTime GetEasterDay(int year)
    {
        int a = year % 19;
        int b = year / 100;
        int c = year % 100;
        int d = b / 4;
        int e = b % 4;
        int f = (b + 8) / 25;
        int g = (b - f + 1) / 3;
        int h = (19 * a + b - d - g + 15) % 30;
        int i = c / 4;
        int k = c % 4;
        int l = (32 + 2 * e + 2 * i - h - k) % 7;
        int m = (a + 11 * h + 22 * l) / 451;
        int n = (h + l - 7 * m + 114) / 31; // This is the month number.
        int p = (h + l - 7 * m + 114) % 31; // This is the date minus one.

        return new DateTime(year, n, p + 1);
    }

    private bool CheckDate(DateTime date, DateTime other)
    {
        return date.Day == other.Day && date.Month == other.Month;
    }

}

Also, some CSS rules are added here for the custom control :

App.css

.flatpickr-day.pink-day {
    background: #ff69b4 !important; /* Hot pink */   
    color: white !important;
    border-radius: 50%;
}

.addon-margin {
    margin-left: 0.25rem !important; /* Adjust as needed */
}

Finally, a screenshot showing how the Custom date picker works. We have highlighted dates in May 2025 here, I chose to generate holidays for +- 20 years here. Obviously, an optimization would be the possibility to only pass in the chose year of the date picker and not a wide range of years. This is however looking to be fast anyways and I believe you can pass in a large number of dates to highlight here. Hopefully, this article has given you more inspiration how to add highlighting of dates into the Blazorise Datepicker component for Blazor !

Pluralizers in Entity Framework

In Entity Framework, you can implement a IPluralizationService to control how entities' names will be pluralized. In additional singularalized. The standard setup is an English pluralizer. In addition, a Spanish pluralizer is available. If you want to support another language for pluralization (and singularization), I have added a sample of this in the following Github repo of mine :

https://github.com/toreaurstadboss/BulkOperationsEntityFramework

Note that the Norwegian Pluralization service could also pluralize words in English and try to check if the word to pluralize is either English or Norwegian. It is about 500,000 English nouns and 100,000, so it might be hard to create a perfect pluralizer for both English or Norwegian. A list of about 40,000+ nouns are available here: https://gist.github.com/trag1c/f74b2ab3589bc4ce5706f934616f6195/ The Norwegian Pluralization service next could use that wordlist to check if the the word is English and use the standard built in English pluralizer service. I will give an updated version of the NorwegianPluralizationService at the end of the article.

NorwegianPluralizationService.cs

using System;
using System.Collections.Generic;
using System.Data.Entity.Infrastructure.Pluralization;
using System.Diagnostics;
using System.Linq;

namespace BulkOperationsEntityFramework.Lib.Services
{

    /// <summary>
    /// Sources for the pluralization rules for Norwegian language:
    /// https://toppnorsk.com/2018/11/18/flertall-hovedregler/
    /// </summary>
    public class NorwegianPluralizationService : IPluralizationService
    {

        public static List<string> PluralizedWords = new List<string>();

        public string Pluralize(string word)
        {
            if (PluralizedWords.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                return word; // Return the already pluralized word
            }

//#if DEBUG
//            Debugger.Break();
//            Debugger.Launch(); // Uncomment this line to break into the debugger when this method is called, for example when database migrations are made with EF Code First
//#endif

            word = NormalizeWord(word);

            string pluralizedWord;

            if (_specialCases.ContainsKey(word))
            {
                pluralizedWord = _specialCases[word];
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            if (_wordsChangingVowelToÆ.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                if (word.Equals("Håndkle", StringComparison.OrdinalIgnoreCase))
                {
                    pluralizedWord = "Håndklær";
                    PluralizedWords.Add(pluralizedWord);
                    return pluralizedWord;
                }
                pluralizedWord = word.Replace("å", "æ").Replace("e", "æ") + "r";
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            if (_wordsForUnits.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                pluralizedWord = word;
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            if (_wordsForRelatives.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                switch (word.ToLower())
                {
                    case "far": pluralizedWord = "Fedre"; break;
                    case "mor": pluralizedWord = "Mødre"; break;
                    case "datter": pluralizedWord = "Døtre"; break;
                    case "søster": pluralizedWord = "Søstre"; break;
                    case "fetter": pluralizedWord = "Fettere"; break;
                    case "onkel": pluralizedWord = "Onkler"; break;
                    case "svigerbror": pluralizedWord = "Svigerbrødre"; break;
                    case "svigerfar": pluralizedWord = "Svigerfedre"; break;
                    case "svigersøster": pluralizedWord = "Svigersøstre"; break;
                    case "svigermor": pluralizedWord = "Svigermødre"; break;
                    case "bror": pluralizedWord = "Brødre"; break;
                    default: pluralizedWord = word; break;
                }
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            if (_wordsNeutralGenderEndingWithEumOrIum.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                if (word.EndsWith("eum"))
                    pluralizedWord = word.Substring(0, word.Length - 3) + "eer";
                else if (word.EndsWith("ium"))
                    pluralizedWord = word.Substring(0, word.Length - 3) + "ier";
                else
                    pluralizedWord = word;
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            if (_wordsNoPluralizationForNeutralGenderOneSyllable.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                pluralizedWord = word;
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            if (_wordChangingVowelsInPluralFemale.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                pluralizedWord = NormalizeWord(word.ToLower().Replace("å", "e").Replace("a", "e") + "er");
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            if (_wordsChangingVowelsInPluralMale.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                string rewrittenWord = NormalizeWord(word.Replace("o", "ø"));
                if (rewrittenWord.Equals("føt", StringComparison.OrdinalIgnoreCase))
                    pluralizedWord = rewrittenWord + "ter";
                else if (rewrittenWord.EndsWith("e"))
                    pluralizedWord = rewrittenWord + "r";
                else if (!rewrittenWord.EndsWith("er"))
                    pluralizedWord = rewrittenWord + "er";
                else
                    pluralizedWord = rewrittenWord;
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            if (_nonEndingWordsInPlural.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                pluralizedWord = word;
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            // General rules
            if (word.EndsWith("er"))
                pluralizedWord = word.Substring(0, word.Length - 2) + "ere";
            else if (word.EndsWith("el"))
                pluralizedWord = word.Substring(0, word.Length - 2) + "ler";
            else if (word.EndsWith("e"))
                pluralizedWord = word + "r";
            else if (word.EndsWith("en"))
                pluralizedWord = word + "er";
            else
                pluralizedWord = word + "er";

            PluralizedWords.Add(pluralizedWord);
            return pluralizedWord;
        }

        public string Singularize(string word)
        {
            word = NormalizeWord(word);

            // Reverse special cases
            var specialSingular = _specialCases.FirstOrDefault(kvp => kvp.Value.Equals(word, StringComparison.OrdinalIgnoreCase));
            if (!specialSingular.Equals(default(KeyValuePair<string, string>)))
                return specialSingular.Key;

            // Words that are the same in singular and plural
            if (_nonEndingWordsInPlural.Contains(word, StringComparer.OrdinalIgnoreCase) ||
                _wordsNoPluralizationForNeutralGenderOneSyllable.Contains(word, StringComparer.OrdinalIgnoreCase) ||
                _wordsForUnits.Contains(word, StringComparer.OrdinalIgnoreCase))
                return word;

            // Irregulars and vowel changes (expand as needed)
            if (word.Equals("Bøker", StringComparison.OrdinalIgnoreCase)) return "Bok";
            if (word.Equals("Føtter", StringComparison.OrdinalIgnoreCase)) return "Fot";
            if (word.Equals("Brødre", StringComparison.OrdinalIgnoreCase)) return "Bror";
            if (word.Equals("Menn", StringComparison.OrdinalIgnoreCase)) return "Mann";
            if (word.Equals("Kvinner", StringComparison.OrdinalIgnoreCase)) return "Kvinne";
            if (word.Equals("Gutter", StringComparison.OrdinalIgnoreCase)) return "Gutt";
            if (word.Equals("Netter", StringComparison.OrdinalIgnoreCase)) return "Natt";
            if (word.Equals("Tær", StringComparison.OrdinalIgnoreCase)) return "Tå";
            if (word.Equals("Tenner", StringComparison.OrdinalIgnoreCase)) return "Tann";
            if (word.Equals("Trær", StringComparison.OrdinalIgnoreCase)) return "Tre";
            if (word.Equals("Knær", StringComparison.OrdinalIgnoreCase)) return "Kne";
            if (word.Equals("Bønder", StringComparison.OrdinalIgnoreCase)) return "Bonde";
            if (word.Equals("Hender", StringComparison.OrdinalIgnoreCase)) return "Hand";
            if (word.Equals("Døtre", StringComparison.OrdinalIgnoreCase)) return "Datter";
            if (word.Equals("Fedre", StringComparison.OrdinalIgnoreCase)) return "Far";
            if (word.Equals("Mødre", StringComparison.OrdinalIgnoreCase)) return "Mor";
            if (word.Equals("Søstre", StringComparison.OrdinalIgnoreCase)) return "Søster";
            if (word.Equals("Øyne", StringComparison.OrdinalIgnoreCase)) return "Øye";

            // "ler" ending (from "el")
            if (word.EndsWith("ler"))
            {
                return word.Substring(0, word.Length - 2);
            }
            if (word.EndsWith("ter"))
            {
                return word.Substring(0, word.Length - 1);
            }

            // "ere" ending (from "er" ending in singular, e.g. "Lærere" -> "Lærer")
            if (word.EndsWith("ere"))
                return word.Substring(0, word.Length - 1);

            // "er" ending (general case, e.g. "Biler" -> "Bil", "Stoler" -> "Stol", "Jenter" -> "Jente")
            if (word.EndsWith("er"))
                return word.Substring(0, word.Length - 2);

            // "r" ending (from "e" ending in singular, e.g. "Jenter" -> "Jente" already handled above)
            if (word.EndsWith("r"))
            {
                var possibleSingular = word.Substring(0, word.Length - 1);
                return possibleSingular;
            }

            // Default: return as is
            return word;
        }

        /// <summary>
        /// Make the world normalized, i.e. first letter upper case and rest lower case letters, the word is trimmed.
        /// Not considering using invariant culture here, as this is a Norwegian pluralization service.
        /// </summary>
        /// <remarks>In case an empty word (null or empty) is passed in, just return the word.
        /// Edge case: In case just One non-empty letter was passed in, make the word also uppercase.</remarks>
        private string NormalizeWord(string word)
        {
            word = word?.Trim();
            if (string.IsNullOrEmpty(word) || word.Trim().Length <= 1) {
                return word?.ToUpper();
            }
            return word.Substring(0, 1).ToUpper() + word.Trim().ToLower().Substring(1);
        }

        private string[] _nonEndingWordsInPlural = new string[] {
            "mus", "sko", "ski", "feil", "ting" }; // Add more non-ending words in plural as needed

        private string[] _wordsChangingVowelsInPluralMale = new string[]
        {
            "bonde", "fot", "bok", "bot", "rot"
        };

        private Dictionary<string, string> _specialCases = new Dictionary<string, string>
        {
            { "Mann", "Menn" } , // 'mann' => 'menn'
            { "Barn", "Barn" }, // 'barn' => 'barn' (no pluralization)
            { "Øye", "Øyne" }, // 'øye' => 'øyne' (plural form of 'eye') //consider adding more special cases here in case all the other pluralization rules do not cover the given word
        };

        private string[] _wordsChangingVowelToÆ = new string[]
        {
            "Håndkle", "Kne", "Tre", "Tå"
        };

        private string[] _wordsForUnits = new string[]
        {
            "meter", "centimeter", "millimeter", "kilometer", "gram", "kilogram", "tonn", "liter", "desiliter", "centiliter", "dollar", "lire",
            "pesetas", "euro", "yen", "franc", "pund", "rupee", "ringgit", "peso", "real", "won", "yuan"
        };

        private string[] _wordChangingVowelsInPluralFemale = new string[]
        {
            "and", "hand", "hånd", "natt", "stang", "strand", "tang", "tann"
        };

        private string[] _wordsForRelatives = new string[]
        {
            "far", "mor", "datter", "fetter", "onkel", "bror", "svigerbror", "svigerfar", "svigermor", "svigersøster", "søster"
        };

        private string[] _wordsNoPluralizationForNeutralGenderOneSyllable = new string[]
        {
            "hus", "fjell", "blad"
        };

        private string[] _wordsNeutralGenderEndingWithEumOrIum = new string[]
        {
            "museum", "Jubileum", "kjemikalium"
        };

    }
}

The following DbConfiguration set up for the DbContext sets up the pluralization service to use for Entity Framework.

ApplicationDbConfiguration.cs

using BulkOperationsEntityFramework.Lib.Services;
using System;
using System.Data.Entity;
using System.Data.Entity.SqlServer;

namespace BulkOperationsEntityFramework
{
    public class ApplicationDbConfiguration : DbConfiguration
    {

        public ApplicationDbConfiguration()
        {
        
            SetPluralizationService(new NorwegianPluralizationService());  //Set up the NorwegianPluralizationService as the Pluralizer         
            
            //more code etc..
        }

    }

}

I have also created a Schema attribute to control schema names of tables convention previously in the solution, so the Norwegian pluralizer is also being used there.

SchemaConvention.cs

using BulkOperationsEntityFramework.Attributes;
using BulkOperationsEntityFramework.Lib.Services;
using System.Data.Entity.ModelConfiguration.Conventions;
using System.Reflection;

namespace BulkOperationsEntityFramework.Conventions
{
    public class SchemaConvention : Convention
    {
        public SchemaConvention()
        {
            var pluralizer = new NorwegianPluralizationService();

            Types().Configure(c =>
            {
                var schemaAttr = c.ClrType.GetCustomAttribute<SchemaAttribute>(false);
                var tableName = pluralizer.Pluralize(c.ClrType.Name);

                if (schemaAttr != null && !string.IsNullOrEmpty(schemaAttr.SchemaName))
                {
                    c.ToTable(tableName, schemaAttr.SchemaName ?? "dbo");
                }
                else
                {
                    c.ToTable(tableName);
                }
            });
        }
    }
}

The DbContext will use the IPluralizationService. Consider first this example DbContext :

ApplicationDbContext

using BulkOperationsEntityFramework.Conventions;
using BulkOperationsEntityFramework.Models;
using BulkOperationsEntityFramework.Test;
using System;
using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Common;
using System.Data.Entity;
using System.Data.Entity.Infrastructure.Interception;
using System.Linq;

namespace BulkOperationsEntityFramework
{

    [DbConfigurationType(typeof(ApplicationDbConfiguration))]
    public class ApplicationDbContext : DbContext
    {       

        public ApplicationDbContext(DbConnection connection) : base(connection, false)
        {
        }

        public ApplicationDbContext() : base("name=App")
        {
        }

        public virtual DbSet Bruker { get; set; }

        public DbSet ArkivertBruker { get; set; }

        public DbSet ArkivertGjest { get; set; }

        public DbSet Sesjon { get; set; }

        public DbSet Jubileum { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {            
            modelBuilder.Conventions.Add(new SchemaConvention());
            //more code etc
          
        }

    }

}

The following test cases checks how good the pluralizer works.

ApplicationDbContextTests.cs

using Bogus;
using BulkOperationsEntityFramework.Lib.Services;
using BulkOperationsEntityFramework.Models;
using FluentAssertions;
using Moq;
using Newtonsoft.Json;
using NUnit.Framework;
using System;
using System.Collections.Generic;
using System.Data.Common;
using System.Data.Entity;
using System.Data.Entity.Infrastructure.Interception;
using System.Diagnostics;
using System.Linq;
using System.Threading.Tasks;

namespace BulkOperationsEntityFramework.Test
{

    [TestFixture]
    public class ApplicationDbContextTests
    {

       
        [Test]
        [TestCaseSource(nameof(NorwegianPluralizationCases))]
        public void CanUsePluralizationService(string word, string expected)
        {
            var norwegianPluralizationService = new NorwegianPluralizationService();
            string pluralizedWord = norwegianPluralizationService.Pluralize(word);
            pluralizedWord.Should().Be(expected, "Norwegian Pluralization service should return the correct plural form of the word.");
        }

        [Test, TestCaseSource(nameof(NorwegianSingularizationCases))]
        public void NorwegianPluralizationService_CanSingularize(string plural, string expectedSingular)
        {
            var norwegianPluralizationService = new NorwegianPluralizationService();

            var actual = norwegianPluralizationService.Singularize(plural);
            Assert.That(actual, Is.EqualTo(expectedSingular), $"Expected singular of '{plural}' to be '{expectedSingular}', but got '{actual}'.");
        }

        public static IEnumerable<TestCaseData> NorwegianPluralizationCases
        {
            get
            {
                yield return new TestCaseData("Bil", "Biler");
                yield return new TestCaseData("Bok", "Bøker");
                yield return new TestCaseData("Hund", "Hunder");
                yield return new TestCaseData("Stol", "Stoler");
                yield return new TestCaseData("Jente", "Jenter");
                yield return new TestCaseData("Gutt", "Gutter");
                yield return new TestCaseData("Lærer", "Lærere");
                yield return new TestCaseData("Barn", "Barn");
                yield return new TestCaseData("Fjell", "Fjell");
                yield return new TestCaseData("Sko", "Sko");
                yield return new TestCaseData("Ting", "Ting");
                yield return new TestCaseData("Mann", "Menn");
                yield return new TestCaseData("Kvinne", "Kvinner");
                yield return new TestCaseData("Bror", "Brødre");
                yield return new TestCaseData("Far", "Fedre");
                yield return new TestCaseData("Mor", "Mødre");
                yield return new TestCaseData("Datter", "Døtre");
                yield return new TestCaseData("Søster", "Søstre");
                yield return new TestCaseData("Øye", "Øyne");
                yield return new TestCaseData("Hand", "Hender");
                yield return new TestCaseData("Fot", "Føtter");
                yield return new TestCaseData("Tå", "Tær");
                yield return new TestCaseData("Tann", "Tenner");
                yield return new TestCaseData("Natt", "Netter");
                yield return new TestCaseData("Tre", "Trær");
                yield return new TestCaseData("Kne", "Knær");
                yield return new TestCaseData("Bonde", "Bønder");

                // _nonEndingWordsInPlural
                yield return new TestCaseData("Mus", "Mus");
                yield return new TestCaseData("Ski", "Ski");
                yield return new TestCaseData("Feil", "Feil");

                // _wordsChangingVowelsInPluralMale
                yield return new TestCaseData("Bot", "Bøter");
                yield return new TestCaseData("Rot", "Røter");

                // _wordsChangingVowelToÆ
                yield return new TestCaseData("Håndkle", "Håndklær");
                yield return new TestCaseData("Kne", "Knær");

                // _wordsForUnits (should not pluralize)
                yield return new TestCaseData("Meter", "Meter");
                yield return new TestCaseData("Gram", "Gram");
                yield return new TestCaseData("Dollar", "Dollar");

                // _wordChangingVowelsInPluralFemale
                yield return new TestCaseData("And", "Ender");
                yield return new TestCaseData("Hånd", "Hender");
                yield return new TestCaseData("Stang", "Stenger");
                yield return new TestCaseData("Strand", "Strender");
                yield return new TestCaseData("Tang", "Tenger");
                yield return new TestCaseData("Tann", "Tenner");

                // _wordsForRelatives (some already covered, but add missing)
                yield return new TestCaseData("Fetter", "Fettere");
                yield return new TestCaseData("Onkel", "Onkler");
                yield return new TestCaseData("Svigerbror", "Svigerbrødre");
                yield return new TestCaseData("Svigerfar", "Svigerfedre");
                yield return new TestCaseData("Svigermor", "Svigermødre");
                yield return new TestCaseData("Svigersøster", "Svigersøstre");

                // _wordsNoPluralizationForNeutralGenderOneSyllable
                yield return new TestCaseData("Hus", "Hus");
                yield return new TestCaseData("Blad", "Blad");

                // _wordsNeutralGenderEndingWithEumOrIum
                yield return new TestCaseData("Museum", "Museer");
                yield return new TestCaseData("Jubileum", "Jubileer");
                yield return new TestCaseData("Kjemikalium", "Kjemikalier");
            }
        }

        public static IEnumerable<TestCaseData> NorwegianSingularizationCases
        {
            get
            {
                yield return new TestCaseData("Biler", "Bil");
                yield return new TestCaseData("Bøker", "Bok");
                yield return new TestCaseData("Hunder", "Hund");
                yield return new TestCaseData("Stoler", "Stol");
                yield return new TestCaseData("Jenter", "Jente");
                yield return new TestCaseData("Gutter", "Gutt");
                yield return new TestCaseData("Lærere", "Lærer");
                yield return new TestCaseData("Barn", "Barn");
                yield return new TestCaseData("Fjell", "Fjell");
                yield return new TestCaseData("Sko", "Sko");
                yield return new TestCaseData("Ting", "Ting");
                yield return new TestCaseData("Menn", "Mann");
                yield return new TestCaseData("Kvinner", "Kvinne");
                yield return new TestCaseData("Brødre", "Bror");
                yield return new TestCaseData("Fedre", "Far");
                yield return new TestCaseData("Mødre", "Mor");
                yield return new TestCaseData("Døtre", "Datter");
                yield return new TestCaseData("Søstre", "Søster");
                yield return new TestCaseData("Øyne", "Øye");
                yield return new TestCaseData("Hender", "Hand");
                yield return new TestCaseData("Føtter", "Fot");
                yield return new TestCaseData("Tær", "Tå");
                yield return new TestCaseData("Tenner", "Tann");
                yield return new TestCaseData("Netter", "Natt");
                yield return new TestCaseData("Trær", "Tre");
                yield return new TestCaseData("Knær", "Kne");
                yield return new TestCaseData("Bønder", "Bonde");
            }
        }

    }
}

The pluralization is used when creating migrations and map entites to table names. The following migration shows how the Norwegian pluralization has pluralized the table names.

202506292302222_Init.cs

namespace BulkOperationsEntityFramework.Migrations
{
    using System.Data.Entity.Migrations;

    public partial class Init : DbMigration
    {
        public override void Up()
        {
            CreateTable(
                "Archive.Arkivertbrukere",
                c => new
                {
                    Id = c.Int(nullable: false, identity: true),
                    Email = c.String(maxLength: 255),
                    FirstName = c.String(maxLength: 255),
                    LastName = c.String(maxLength: 255),
                    PhoneNumber = c.String(maxLength: 255),
                })
                .PrimaryKey(t => t.Id);

            CreateTable(
                "Arkiv.Gjester",
                c => new
                {
                    Id = c.Int(nullable: false, identity: true),
                    Email = c.String(maxLength: 255),
                    FirstName = c.String(maxLength: 255),
                    LastName = c.String(maxLength: 255),
                })
                .PrimaryKey(t => t.Id);

            CreateTable(
                "dbo.Brukere",
                c => new
                {
                    Id = c.Int(nullable: false, identity: true),
                    Email = c.String(maxLength: 255),
                    FirstName = c.String(maxLength: 255),
                    LastName = c.String(maxLength: 255),
                    PhoneNumber = c.String(maxLength: 255),
                })
                .PrimaryKey(t => t.Id);

            CreateTable(
                "dbo.Jubileer",
                c => new
                {
                    Id = c.Int(nullable: false, identity: true),
                    Date = c.DateTime(),
                    Description = c.String(maxLength: 255),
                })
                .PrimaryKey(t => t.Id);

            CreateTable(
                "dbo.Sesjoner",
                c => new
                {
                    Key = c.Guid(nullable: false),
                    CreatedAt = c.DateTime(nullable: false),
                    ExpiresAt = c.DateTime(),
                    IpAddress = c.String(maxLength: 255),
                    UserAgent = c.String(maxLength: 255),
                })
                .PrimaryKey(t => t.Key);

        }

        public override void Down()
        {
            DropTable("dbo.Sesjoner");
            DropTable("dbo.Jubileer");
            DropTable("dbo.Brukere");
            DropTable("Arkiv.Gjester");
            DropTable("Archive.Arkivertbrukere");
        }
    }
}

The table names above are pluralized into their Norwegian pluralization. In Norwegian we call these words "ubestemt flertall", indefinite plural. It is possible to add detection if the passed in noun is an English noun. But beware that there are several Norwegian nouns overlapping these English nouns. So in fairness, a Norwegian wordlist should also be checked. However, the following update only shows how an English dictionary can be checked. The English noun list is put into a static variable into memory for quick access. The world list I have tested with got about 40,0000+ noun as mentioned previously in this article.

NorwegianPluraizationService.cs

(updated with a check against a English noun list and using the default EnglishPluralizationService that Entity Framework contains to add support for English noun pluralization (and singularization)

using System;
using System.Collections.Generic;
using System.Data.Entity.Infrastructure.Pluralization;
using System.IO;
using System.Linq;
using System.Reflection;

namespace BulkOperationsEntityFramework.Lib.Services
{

    /// <summary>
    /// Sources for the pluralization rules for Norwegian language:
    /// https://toppnorsk.com/2018/11/18/flertall-hovedregler/
    /// </summary>
    public class NorwegianPluralizationService : IPluralizationService
    {
        private EnglishPluralizationService _englishPluralizationService = new EnglishPluralizationService();
        
        public static List<string> PluralizedWords = new List<string>();

        private static List<string> EnglishNounsWordList = null;

        public string Pluralize(string word)
        {
            if (PluralizedWords.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                return word; // Return the already pluralized word
            }

            //#if DEBUG
            //            Debugger.Break();
            //            Debugger.Launch(); // Uncomment this line to break into the debugger when this method is called, for example when database migrations are made with EF Code First
            //#endif

            word = NormalizeWord(word);

            if (EnglishNounsWordList == null)
            {
                EnglishNounsWordList = new List<string>();
                var assembly = typeof(NorwegianPluralizationService).Assembly;
                // Adjust the resource name to match your project's default namespace and folder structure
                var resourceName = "BulkOperationsEntityFramework.Lib.Services.EnglishNouns.txt";
                using (var stream = assembly.GetManifestResourceStream(resourceName))
                using (var reader = new StreamReader(stream))
                {
                    EnglishNounsWordList.AddRange(reader.ReadToEnd().Split('\n').Select(l => l.Trim()));
                }
            }

            string pluralizedWord;

            if (EnglishNounsWordList.Contains(word, StringComparer.OrdinalIgnoreCase))
            {
                pluralizedWord = _englishPluralizationService.Pluralize(word);
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }

            if (_specialCases.ContainsKey(word))
            {
                pluralizedWord = _specialCases[word];
                PluralizedWords.Add(pluralizedWord);
                return pluralizedWord;
            }
            
            //more code

To sum up, we can customize Entity Framework pluralization to support other languages. But please note that there is a lot of work to make a good pluralization service. Also, you probably want to support English nouns too. The last version above did not use a Norwegian wordlist. A better pluralization service could directly try to lookup pluralized and singularized nouns of both languages instead of all the general and specific rules presented in these code samples. The English pluralization service do not do this, instead it relies on rule sets, such as the first version of
the Norwegian pluralizer shown earlier in this article is using.