Wednesday, 6 April 2022

Abstract factory - registering factory methods

Today, instead of relying on factories we often resort to dependency injection. However, you can 'blueprint' objects by offering a helper class to register both interfaces with concrete classes and default parameters if you want to instantiate a type of object with given parameters using an abstract factory class The following code shows how we can start building such a registration approach, supporting also unregistering factories and creating objects which are registered. You will have to add thread safety and better checking if a given type T and parameters P1..PN are registered, here is though a simple Abstract factory. Paste the code into Linqpad 6 or Linqpad 7 to test it out !
 void Main()

 var factory = new AbstractFactory(); 
 factory.Register<IDrive>(() => new FoulwheeledDrive());
 factory.Register<Car, string, string, IDrive>((string make, string model, IDrive drive) => new Car(make, model, drive)); 
 var car = factory.Create<Car, string, string, IDrive>("BMW", "M5", factory.Create<IDrive>());
 //can also unregister a factory method
 //factory.Unregister<Car, string, string, IDrive>();
 //var car2 = factory.Create<Car, string, string, IDrive>("BMW", "M5", factory.Create<IDrive>());

public interface IVehicle {
 string Model { get; set; }
 string Make { get; set; }

public interface IDrive {
 string Drive();

public class FoulwheeledDrive : IDrive {
 public string Drive() => "Car driving with four wheeled motion";

public class Car  {

  public string Make { get; set; }  
  public string Model { get; set; }
  public IDrive Drive { get; set; }
  public Car(string model, string make, IDrive drive){
    Model = model; 
 	Make = make;
	Drive = drive;
  public override string ToString() => $"{Model} {Make} : {Drive.Drive()}";

public class AbstractFactory {

 private readonly Dictionary<Type, Delegate> _factories = new (); 
 public void Register<T>(Func<T> factory) => _factories[typeof(Func<T>)] = factory;
 public void Register<T,P>(Func<P,T> factory) => _factories[typeof(Func<P,T>)] = factory;
 public void Register<T,P1, P2>(Func<P1, P2, T> factory) => _factories[typeof(Func<P1, P2, T>)] = factory;
 public void Register<T, P1, P2, P3>(Func<P1, P2, P3, T> factory) => _factories[typeof(Func<P1, P2, P3, T>)] = factory;
 public void Register<T, P1, P2, P3, P4>(Func<T> factory) => _factories[typeof(Func<P1, P2, P3, P4, T>)] = factory;
 public void Register<T, P1, P2, P3, P4, P5>(Func<T> factory) => _factories[typeof(Func<P1, P2, P3, P4, P5, T>)] = factory;
 public void Unregister<T>() => _factories.Remove(typeof(Func<T>)); 
 public void Unregister<T,P>() => _factories.Remove(typeof(Func<P,T>));
 public void Unregister<T,P1, P2>() => _factories.Remove(typeof(Func<P1, P2, T>));
 public void Unregister<T,P1, P2, P3>() => _factories.Remove(typeof(Func<P1, P2, P3, T>)); 
 public void Unregister<T,P1, P2, P3, P4>() => _factories.Remove(typeof(Func<P1, P2, P3, P4, T>));
 public void Unregister<T,P1, P2, P3, P4, P5>() => _factories.Remove(typeof(Func<P1, P2, P3, P4, P5, T>));
 public T Create<T>() => ((Func<T>)_factories[typeof(Func<T>)])();
 public T Create<T, P>(P p) => ((Func<P,T>)_factories[typeof(Func<P,T>)])(p);
 public T Create<T, P1, P2>(P1 p1, P2 p2) => ((Func<P1, P2,T>)_factories[typeof(Func<P1, P2,T>)])(p1, p2);
 public T Create<T, P1, P2, P3>(P1 p1, P2 p2, P3 p3) => ((Func<P1, P2, P3, T>)_factories[typeof(Func<P1, P2, P3, T>)])(p1, p2, p3);
 public T Create<T, P1, P2, P3, P4>(P1 p1, P2 p2, P3 p3, P4 p4) => ((Func<P1, P2, P3, P4, T>)_factories[typeof(Func<P1, P2, P3, P4, T>)])(p1, p2, p3, p4);
 public T Create<T, P1, P2, P3, P4, P5>(P1 p1, P2 p2, P3 p3, P4 p4, P5 p5) => ((Func<P1, P2, P3, P4, P5, T>)_factories[typeof(Func<P1, P2, P3, T>)])(p1, p2, p3, p4, p5);


The registration of the factories then instantiate the instance. Note that we do not use the new operator when instantiating the object. We avoid using Activator.CreateInstance and reflection using this approach. Why use this pattern when we want to create 'blueprint' objects and want to have standard ways of instantiating them and for example avoid using concrete types. Most IoC frameworks allow you to create compound classes and supply parameters, so the abstract factory pattern seems a bit 'crude', but it is used many places still.

Friday, 1 April 2022

GraphQL in Asp.Net Core - Creating a flexible API

More and more .NET Developers have heard about GraphQL. This started as an in-house project in Facebook 2012 to provide a flexible way of sending customized data to mobile clients. Giving the clients the possible to query after tailored data meant sending less data over the wire to the mobiles with less bandwidth. As cell phones moves over to 5G networks, the issue means less and less (in urban areas with good base station coverage), however we should of course seek to always optimize our data transfer as pure bandwidth usage is always a valued thing to optimize. And added dimension is the less cost of creating APIs as we can tailor our data needs. Instead of creating methods for either returning lookup ids and then querying after entire data objects, we can project only the data we need to retrieve to present data on the mobile clients in a meaningful way. Whatever makes your boat rock for showing interest in GraphQL, this article will discuss how you can get started with GraphQL in Asp.Net Core. I have prepared a demo here:
The demo repository shows a list of the tallest mountains in the municipialites in Norway. Norway is a land of mountains and it is always to know which mountain is the very tallest in the municipiality you are visiting! (I enjoy mountain climbing and hiking now and then in my spare time). The demo page shows a text area where you can customize the data to load here. Of course we can only load the data provided for us. We can also use the Ui playground for GraphQL added for us here too:
First off, we need to grab some Nuget packages for GraphQL. We will be using Asp.Net Core 3.1. in this article.
        <PackageReference Include="GraphQL" Version="2.4.0" />
	<PackageReference Include="GraphQL.Server.Transports.AspNetCore" Version="3.4.0" />
	<PackageReference Include="GraphQL.Server.Transports.WebSockets" Version="3.4.0" />
	<PackageReference Include="GraphQL.Server.Ui.Playground" Version="3.5.0-alpha0046" />  
Then we need to specify in our Startup class the needed setup.
using AspNetCore_GraphQLDemo.GraphQL; using AspNetCore_GraphQLDemo.GraphQL.Messaging; using Data; using Data.Repositories; using GraphQL; using GraphQL.Server; using GraphQL.Server.Ui.Playground; using Microsoft.AspNetCore.Builder; using Microsoft.AspNetCore.Diagnostics; using Microsoft.AspNetCore.Hosting; using Microsoft.AspNetCore.Http; using Microsoft.AspNetCore.WebSockets; using Microsoft.EntityFrameworkCore; using Microsoft.Extensions.Configuration; using Microsoft.Extensions.DependencyInjection; using Microsoft.Extensions.Hosting; using Newtonsoft.Json; namespace AspNetCore_GraphQLDemo { public class Startup { private readonly IWebHostEnvironment _env; public Startup(IConfiguration configuration, IWebHostEnvironment env) { _env = env; Configuration = configuration; } public IConfiguration Configuration { get; } // This method gets called by the runtime. Use this method to add services to the container. public void ConfigureServices(IServiceCollection services) { // If using IIS: services.Configure<IISServerOptions>(options => { options.AllowSynchronousIO = true; }); services.AddControllersWithViews(); services.AddHttpContextAccessor(); services.AddRazorPages().AddRazorRuntimeCompilation(); services.AddDbContext<MountainDbContext>(options => { options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")); }); services.AddScoped<IMountainRepository, MountainRepository>(); services.AddScoped<IDependencyResolver>(s => new FuncDependencyResolver(s.GetRequiredService)); services.AddScoped<MountainSchema>(); services.AddSingleton<MountainMessageService>(); services.AddSingleton<MountainDetailsDisplayedMessageService>(); services.AddGraphQL(x => { x.EnableMetrics = true; x.ExposeExceptions = _env.IsDevelopment(); x.SetFieldMiddleware = true; }).AddGraphTypes(ServiceLifetime.Scoped) .AddUserContextBuilder(httpContext => httpContext.User) .AddDataLoader() .AddWebSockets(); services.AddCors(options => { options.AddPolicy(name: "MyAllowSpecificOrigins", builder => { builder.AllowAnyOrigin().AllowAnyMethod(); }); }); } //static IEnumerable<Type> GetGraphQlTypes() //{ // return typeof(Startup).Assembly // .GetTypes() // .Where(x => !x.IsAbstract && // (typeof(IObjectGraphType).IsAssignableFrom(x) || // typeof(IInputObjectGraphType).IsAssignableFrom(x))); //} // This method gets called by the runtime. Use this method to configure the HTTP request pipeline. public void Configure(IApplicationBuilder app, IWebHostEnvironment env) { if (env.IsDevelopment()) { app.UseDeveloperExceptionPage(); app.UseBrowserLink(); } app.UseExceptionHandler(errorApp => { errorApp.Run(async context => { context.Response.Redirect("/Error"); context.Response.StatusCode = 500; var exceptionHandlerPathFeature = context.Features.Get<IExceptionHandlerPathFeature>(); var exception = exceptionHandlerPathFeature.Error; var result = JsonConvert.SerializeObject(new { error = exception.Message }); context.Response.ContentType = "application/json"; await context.Response.WriteAsync(result); }); }); app.UseStaticFiles(); app.UseRouting(); app.UseCors("MyAllowSpecificOrigins"); app.UseWebSockets(); app.UseGraphQLWebSockets<MountainSchema>("/graphql"); //app.UseAuthorization(); app.UseEndpoints(endpoints => { endpoints.MapDefaultControllerRoute(); }); app.UseGraphQL<MountainSchema>(); if (env.IsDevelopment()) { app.UseGraphQLPlayground(new GraphQLPlaygroundOptions { }); } } } }
In ConfigureServices method above we register the schema for our GraphQL method.
We also add GraphQL itself and setup also web sockets (which are needed for GraphQL).
       services.AddGraphQL(x =>
                    x.EnableMetrics = true; x.ExposeExceptions = _env.IsDevelopment(); x.SetFieldMiddleware = true; }).AddGraphTypes(ServiceLifetime.Scoped)
            .AddUserContextBuilder(httpContext => httpContext.User)
Just as a side note, you want to add Cors also:
     services.AddCors(options =>
                options.AddPolicy(name: "MyAllowSpecificOrigins",
                    builder =>
Inside Configure method we add also the following to enable GraphQL:




            app.UseEndpoints(endpoints =>

            if (env.IsDevelopment())
                app.UseGraphQLPlayground(new GraphQLPlaygroundOptions
Our Mountainschema looks like this:
using AspNetCore_GraphQLDemo.GraphQL.Types; using AspNetCore_GraphQLDemo.GraphQL.Types.Directives; using GraphQL; using GraphQL.Instrumentation; using GraphQL.Types; namespace AspNetCore_GraphQLDemo.GraphQL { public class MountainSchema : Schema { public MountainSchema(IDependencyResolver resolver) : base(resolver) { Query = resolver.Resolve<MountainQuery>(); Mutation = resolver.Resolve<MountainMutation>(); Subscription = resolver.Resolve<MountainSubscription>(); RegisterDirective(new LowercaseDirective()); RegisterDirective(new OrderbyDirective()); var builder = new FieldMiddlewareBuilder(); builder.Use<LowercaseFieldsMiddleware>(); builder.ApplyTo(this); builder.Use(next => { return context => { return next(context).ContinueWith(x => { var c = context; var result = x.Result; result = OrderbyQuery.OrderIfNecessary(context, result); return result; }); }; }); builder.ApplyTo(this); //builder.Use<CustomGraphQlExecutor<MountainSchema>>(); //builder.ApplyTo(this); } } }
We pass in a IDependencyResolver (dependency!) into the constructor and resolve the classes we desire (we inherit from Schema class). We wire up our schema here to the Query, Mutation and Subscription we desire and register directives. Here is how the Query property is set:
using AspNetCore_GraphQLDemo.GraphQL.Types; using Data; using Data.Repositories; using GraphQL.Types; namespace AspNetCore_GraphQLDemo.GraphQL { public class MountainQuery : ObjectGraphType { public MountainQuery(IMountainRepository mountainRepository) { Field<ListGraphType<MountainType>>("mountains", resolve: context => mountainRepository.GetAll() ); FieldAsync<MountainType>("mountain", arguments: new QueryArguments(new QueryArgument<NonNullGraphType<MountainIdInputType>> {Name = "id"}), resolve: async context => { var mountain = context.GetArgument<MountainInfo>("id"); var mountainFromDb = await mountainRepository.GetById(mountain.Id); return mountainFromDb; }); //FieldAsync<MountainType>("selectmountain", // arguments: new QueryArguments(new QueryArgument(typeof(int)) { Name = "id" }), // resolve: async context => // { // var mountain = context.GetArgument<MountainInfo>("id"); // var mountainFromDb = await mountainRepository.GetById(mountain.Id); // return mountainFromDb; // }); //sadly, we need to inherit from IGraphType and cannot just have simple scalar arguments in GraphQL.Net.. } } }
As you can see, we can define multiple queries. We inherit from ObjectGraphType and pass in a IMountainRepository. This is an interface for your repository, which fetches data via Entity Framework Core and you can then load data into GraphQL from the local database (The DEMO uses Sql Server (SQLEXPRESS)) via EF Core in a simple manner by only providing the repo via dependency injection. We define via the methods Field and FieldAsync our methods (note the use of string constants as a string value we can use in GraphQL queries of ours that resides in the Schema) and the resolve lambda tells how data is to be fetched. We can specify arguments also. The "mountain" FieldAsync method also accepts arguments via the
arguments lambda and this allows us parameterized access to our data. Over to the Subscription property. It looks like this:
using AspNetCore_GraphQLDemo.GraphQL.Messaging;
using AspNetCore_GraphQLDemo.GraphQL.Types;
using GraphQL.Resolvers;
using GraphQL.Types;

namespace AspNetCore_GraphQLDemo.GraphQL
    public class MountainSubscription : ObjectGraphType
        public MountainSubscription(MountainDetailsDisplayedMessageService mountainDetailsDisplayedMessageService)
            Name = "Subscription";
            AddField(new EventStreamFieldType
                Name = "detailsDisplayed",
                Type = typeof(MountainDetailsMessageType),
                Resolver = new FuncFieldResolver<MountainDetailsMessage>(c => c.Source as MountainDetailsMessage),
                Subscriber = new EventStreamResolver<MountainDetailsMessage>(c => mountainDetailsDisplayedMessageService.GetMessages())
Here we inherit from ObjectGraphType (as we did for Query) and we use the MountainDetailsDisplayedMessageService. This was added as a (concrete class) singleton in the Startup.cs file. The message service uses RxJs serverside to handle the Pub-sub pattern of the subscriber. We are using System.Reactive.Subjects here.
using System; using System.Reactive.Linq; using System.Reactive.Subjects; namespace AspNetCore_GraphQLDemo.GraphQL.Messaging { public class MountainDetailsDisplayedMessageService { private readonly ISubject<MountainDetailsMessage> _messageStream = new ReplaySubject<MountainDetailsMessage>(1); public MountainDetailsMessage AddMountainDetailsMessage(int id) { var message = new MountainDetailsMessage { Id = id }; _messageStream.OnNext(message); return message; } public IObservable<MountainDetailsMessage> GetMessages() { return _messageStream.AsObservable(); } } }
The mutation looks like this:
using AspNetCore_GraphQLDemo.GraphQL.Messaging; using AspNetCore_GraphQLDemo.GraphQL.Types; using Data; using Data.Repositories; using GraphQL.Types; namespace AspNetCore_GraphQLDemo.GraphQL { public class MountainMutation : ObjectGraphType { public MountainMutation(IMountainRepository mountainRepository, MountainMessageService mountainMessageService) { FieldAsync<MountainType>("createMountain", arguments: new QueryArguments( new QueryArgument<NonNullGraphType<MountainInputType>> {Name = "mountain"}), resolve: async context => { var mountain = context.GetArgument<MountainInfo>("mountain"); await mountainRepository.AddMountain(mountain); mountainMessageService.AddMountainAddedMessage(mountain); return mountain; }); FieldAsync<MountainType>("removeMountain", arguments: new QueryArguments( new QueryArgument<NonNullGraphType<MountainIdInputType>> { Name = "id" }), resolve: async context => { var mountain = context.GetArgument<MountainInfo>("id"); await mountainRepository.RemoveMountain(mountain.Id); return mountain; }); } } }
We can create a mountain like this in GraphQL Query:
 mutation {
  createMountain(mountain: {
   county: "Svalbard"
  muncipiality: "Svalbard"
  officialName: "Newtontoppen"
  referencePoint: "Isbjønn på toppen"
  comments: "Husk rask snøskuter",
  metresAboveSeaLevel: "1713",
  primaryFactor: "1713"
  }) {    
And we can remove a mountain (don't we all?) like this:

# Write your query or mutation here
mutation {
  removeMountain(id: {
    id: 370
  }) { id }
If you clone the repo you will find more source code concerning directives such as lowercase and sorting. As you saw in MountainSchema I use the FieldMiddlewareBuilder to do the sorting as this needs to tap into the pipeline more of GraphQL.Net. We also need some more code - for the client side of course. The client side code relies on Apollo Client lib like this:
<script src=""></script>
The libman.json file (the similar file to package.json when it comes to specifying client-side libraries in .net core mvc solutions) of the demo solution looks like this I have used looks like this:
{ "version": "1.0", "defaultProvider": "cdnjs", "libraries": [ { "library": "twitter-bootstrap@4.2.1", "destination": "wwwroot/lib/bootstrap", "files": [ "js/bootstrap.bundle.js", "css/bootstrap.min.css" ] }, { "library": "jquery@3.3.1", "destination": "wwwroot/lib/jquery", "files": [ "jquery.min.js" ] }, { "provider": "unpkg", "library": "font-awesome@4.7.0", "destination": "wwwroot/lib/font-awesome/" }, { "provider": "unpkg", "library": "toastr@2.1.4", "destination": "wwwroot/lib/toastr/" } ] }
We then need some client side code to load data from GraphQL server of ours.

    function LoadGraphQLDataIntoUi(result) {

        var tableBody = $("#mountainsTableBody");

        var tableHeaderRow = $("#mountainsTableHeaderRow");

        var rowIndex = 0; => {

            if (rowIndex == 0) {
                Object.keys(mountain).forEach(key => {
                    if (key === '__typename') {


            Object.keys(mountain).forEach(key => {
                if (key === '__typename') {
                if (key === 'id') {
                    tableBody.append(`<td><a href='/home/mountaindetails/?id=${mountain[key]}'><i class='fa fa-arrow-right'></i></a> ${mountain[key]}</td>`);





        toastr.success('Loaded GraphQL data from server into the UI successfully.');


    $("#btnConnect").click(function () {


    $("#btnLoadData").click(function () {
        var gqlQueryContents = $("#GraphQLQuery").val();
        LoadGraphQLData(gqlQueryContents, LoadGraphQLDataIntoUi);'Retrieving data from API using GraphQL.');

    $(document).ready(function () {


        var initialQuery = `
                    mountains {
                        fylke: county
                        kommune: muncipiality
                        hoydeOverHavet: calculatedMetresAboveSeaLevel
                        offisieltNavn: officialName
                        primaerfaktor: calculatedPrimaryFactor
                        referansePunkt: referencePoint



And then a method using Apollo client lib to load the data:
 * Loads GraphQL data specified by query expression and passes the 'result' array to the callBackFunction
 * callBackFunction should be Js method (function) that accept one parameter, preferably called result, which is an object
 * that contains a object.
function LoadGraphQLData(gqlQuery, callBackFunction) {

    var apolloClient = new Apollo.lib.ApolloClient({
        networkInterface: Apollo.lib.createNetworkInterface({
            uri: 'http://localhost:2542/graphql',
            transportBatching: true,
        }), connectToDevTools: true
    var query = Apollo.gql(gqlQuery);

        query: query,
        variables: {}
    }).then(result => {
    }).catch(error => {
        toastr.error(error, 'GraphQL loading failed');

Saturday, 19 March 2022

Using C# 9 language features in .NET Framework and .NET Standard projects

C# 7.0 came out in March 2017 and Microsoft has published other frameworks later, such as .NET Core and .NET 5 plus .NET 6. If you are working with a .NET Framework based solution (or .NET Standard 2.0), you can actually get support for C# 8 and C# 9 language version - enabling you to utilize more of C# language features. The following steps can be used to enable C# language 9 in for example .NET Framework 4.8 (tested and verified that I could use records (a C# 9 language feature).
  • Specify in the .csproj file(s) that you want to use <LangVersion> element set to 9.0
  • Consider using a file called Directory.Build.props (at the root level of your solution) (Case sensitive on Linux) with this shared setting to enabled C# 9.0 version in all projects.
  • Using C# 9 language version also requires you to include a small file in each project listed below, call it IsExternalInitPatch.cs for example.
File IsExternalInitPatch.cs should include this :

namespace System.Runtime.CompilerServices
    internal static class IsExternalInit { }

Now you can start playing around with C# 9 in a .NET Framework 4.8 solution for example, which earlier has been limited to C# 7.1 and no later language version features of C#.

namespace SomeAcme.SomeProduct.Common.Test
    /// <summary>
    /// This is just a test of csharp 9 for SomeAcme.SomeProduct
    /// Note that Directory.Build.props in this branch uses LangVersion set to 9.0 and we need the file IsExternalInit.cs in every project
    /// </summary>
    /// <remarks>
    /// See these two urls: 
    /// </remarks>
    public class TestOutCsharpNine

        public record Operasjon (DateTime StartTid, bool ErElektiv, string PasientNavn);

        public void Test_Records_ChsharpNine_And_Deconstruction_And_Discardable_Variables()
            var op = new Operasjon(DateTime.Today.AddHours(8).AddMinutes(15), true, "Bjarne Brøndbo");
            (_, _, string pasientNavn) = op;

        public void Test_Init_Only_Props()
            var op = new OperasjonWithInitOnlyProps
                ErElektiv = true,
                PasientNavn = "Thomas Brøndbo"
            // op.PasientNavn = "foo"; 
            //uncommenting line above should demonstrate init only property giving compiller error if trying to mutate or alter this property

        public class OperasjonWithInitOnlyProps
            public string PasientNavn { get; init; }
            public bool ErElektiv { get; init; }

The CSharp compiler sets up default the CSharp language features according to these rules: The compiler determines a default based on these rules:
Target framework	version	C# language version default
.NET	6.x	C# 10
.NET	5.x	C# 9.0
.NET Core	3.x	C# 8.0
.NET Core	2.x	C# 7.3
.NET Standard	2.1	C# 8.0
.NET Standard	2.0	C# 7.3
.NET Standard	1.x	C# 7.3
.NET Framework	all	C# 7.3
So .NET Framework and .NET Standard based solution has not gotten per default any modernization of C# sharp features since March 2017 (five years ago), but we can with some small modification still use C# 9.0 which came out 1.5 years ago. Of course, this C# language version is meant to be used with .NET 5, so do not expect everything to be supported on it. However, chances are high that much of C# 8 and C# 9 language features could be handy to use in many .NET Framework and .NET Standard based projects. For example, records with their support for immutability is definately a big new thing in C# compared to what is avilable in C# language version 8 or earlier. Lastly, you must also consider how to build C# 9 language features (which assumes .NET 5 available) on a build server. For Team City for example, you must install .NET 5 SDK on the build agent.
Also, most likely you have for example a MS Build step in Team City, so you should use MS Build 16 (VS 2019 Build tools) and install the build tools for VS 2019 on the build agent from this url or google for Build Tools for VS 2019: For Azure Devops, choose the VS 2022 agent. I still had to add a "Use NET Core" task and choose 'Package to install' set to 'SDK (contains runtime)', the YAML looks like this:

- task: UseDotNet@2
  displayName: 'Use .NET Core sdk 5.0.100'
    version: 5.0.100
    includePreviewVersions: true

Also note this - albeit you might have .NET Framework 4.8 in a project, your config file like app.config might have this :

<?xml version="1.0" encoding="utf-8" ?>
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
The supportedRuntime might force you to in a specific project to have a LangVersion set to a lower value anyways. So you might need for example to have LangVersion set to 7.1 in one project and default to LangVersion 9.0. To sum up :
  • .NET Framework and .NET Standard can still use C# language version 8 or 9. You need to do the adjustments I mentioned in this article.
  • C# language version 10 is only supported by .NET 6. To use this language version you have to upgrade framework..
  • Also - test out new language features in one project first and use the basic features first. If you use advanced language features of C# language version 8 or 9 you might consider some glitches.. However, you should get a compiler warning for most errors you encounter.
  • Don't forget that your build agent must be able to build the solution too. So you can use VS 2022 hosted agent and consider also the USE NET Core Sdk task I mentioned here if you build in Azure Devops. If you use a self-hosted agent, like Team City on-premises build agent, you need to install the newest VS 2019 SDK / Build Tools to ensure that you have the C# langversion.
In the Developer command prompt on the build agent you can run this command
  csc -langversion:? 
This should output the langversions of C# your build agent supports. It also works on a developer PC (use the VS 2019 command prompt). As I noted, C# 10 is only supported in .NET 6. We might have a future situation where C# 11 is still supported in a .NET 6 solution - I am not sure what Microsoft is planning here. But for other and earlier frameworks, it looks like C# 9 is the end of the road of language versions - we have to upgrade to .NET 6 to utilize newer language features (or consider dragging in Nuget compiler packages ..)