Looking into (circular) dependencies of MEF using C# and Ndepend - Migrating from MEF to Autofac

I decided to look into circular dependencies in C# using reflection and NDepend today. A circular dependency is problematic, especially if you are using dependency injection. In fact, if your system injects dependencies through constructors, if part A imports part B and vice versa - you will usually get a crash. To instantiate the part A we need part B, but that holds also for part A instantiating part B. A huge system I have been working on for years uses MEF or Managed Extensibility Framework. This is primarily an extension framework for allowing pluggable applications, such as seen in Silverlight. It also provides Inversion of Control and you can also import through constructors, that is - every type can decorate one constructor with the [ImportingConstructor] attribute. My system however uses property based injection. You can decorate a property with [Import] attribute and the type of the constructor will then create an instance of an object exporting itself as that type. You can import either concrete or interface based types, and it is also possible to specify a key for the import (string identifier). So the bad part about property based injections is that circular injections can creep up on you - the system will not crash - but it allows circular depedendencies to exist in your system. I created the following Unit Test to detect these circular dependencies.

        [Test]
        [Category(TestCategories.IntegrationTest)]
        public void OutputCircularDependencies()
        {
            var compositionParts = new List>CompositionPart<();
          
            foreach (var part in _aggregateCatalog.Parts)
            {
                var importList = new List>string<();
               
                foreach (var import in part.ImportDefinitions)
                {
                    if (import != null)
                    {
                        importList.Add(import.ContractName);
                    }
                }

                foreach (var export in part.ExportDefinitions)
                {
                    string exportType = null;
                    if (export.Metadata.ContainsKey("ExportTypeIdentity"))
                        exportType = export.Metadata["ExportTypeIdentity"].ToString();

                    string creationPolicy = null;
                    if (export.Metadata.ContainsKey("System.ComponentModel.Composition.CreationPolicy"))
                        creationPolicy = export.Metadata["System.ComponentModel.Composition.CreationPolicy"].ToString();
                    compositionParts.Add(new CompositionPart(part.ToString(), exportType, creationPolicy, importList));                   
                }             
            }

            foreach (var part in compositionParts)
            {
                //check each import if it imports this part
                foreach (var importPart in part.Imports)
                {
                    var matchingPart = compositionParts.FirstOrDefault(c =< c.Identity == importPart);
                    if (matchingPart != null)
                    {
                        if (matchingPart.Imports.Any(i =< i == part.Identity))
                        {
                            //Circular reference detected!
                            Console.WriteLine(@"Component {0} is circular dependent of component {1}", part.Name, matchingPart.Name);
                        }
                    }
                }
            }

            
        }

The code loops through the ComposablePart parts of the AggregateCatalog.Each part has ExportDefinitions (usually one export) and ImportDefinitions (often multiple imports). The first pass will then just gather up information for all the parts of the AssemblyCatalog and then loop through each part again and loop though its imports in an inner loop. If the imported part imports the part itself, we have a circular dpeendency. The test just outputs the circular dependencies to the console. I use the class CompositionPart to have a entity to contain some information about each composition part of the AssemblyCatalag instance. It is just a regular AssemblyCatalog in MEF, created like this:

        readonly AggregateCatalog _aggregateCatalog = new AggregateCatalog();
        CompositionContainer _container;

        [SetUp]
        public void CommonInitialize()
        {
            _aggregateCatalog.Catalogs.Add(new AssemblyCatalog(System.Reflection.Assembly.GetAssembly(typeof(SomeFeatureModule))));
            _aggregateCatalog.Catalogs.Add(new AssemblyCatalog(System.Reflection.Assembly.GetAssembly(typeof(SomeFeatureServiceAgent))));
            _aggregateCatalog.Catalogs.Add(new AssemblyCatalog(System.Reflection.Assembly.GetAssembly(typeof(SomeCommonUtil))));
            _aggregateCatalog.Catalogs.Add(new AssemblyCatalog(System.Reflection.Assembly.GetAssembly(typeof(SomeProvider))));           
            _container = new CompositionContainer(_aggregateCatalog);
            _container.ComposeParts();
        }

As we can see, the AggregateCatalog just consists of several assemblies in .NET. I rewrite the software to not use MEF and property based imports (i.e. properties decorated with the import attribute of System.ComponentModel.Composition.ImportAttribute) by looping through the parts of the AggregateCatalog and looking at the export definitions. The target is Autofac, which is a DI framework that offers dependency injection IMHO better than MEF ImportingConstructor, so analyzing the export defiitions, I can create a list of statements to migrate from MEF to Autofac.

        [Test]
        [Category(TestCategories.IntegrationTest)]
        public void OutputAutofacRegistrations()
        {

            var sb = new StringBuilder();

            foreach (var part in _aggregateCatalog.Parts)
            {
                string partName = part.ToString();

                if (part.ExportDefinitions != null)
                {
                    foreach (var export in part.ExportDefinitions)
                    {
                        string exportType = null;
                        if (export.Metadata.ContainsKey("ExportTypeIdentity"))
                             exportType = export.Metadata["ExportTypeIdentity"].ToString();                        
                        
                        string creationPolicy = null;
                        if (export.Metadata.ContainsKey("System.ComponentModel.Composition.CreationPolicy"))
                            creationPolicy = export.Metadata["System.ComponentModel.Composition.CreationPolicy"].ToString();
                        
                        string autofacExportDefinition = string.Format("builder.RegisterType>{0}<(){1}{2};", partName, 
                            !string.IsNullOrEmpty(exportType) ? ".As>" + exportType + "<()" : string.Empty,
                            !string.IsNullOrEmpty(creationPolicy) && creationPolicy.ToLower().Contains("nonshared") ? 
                            ".InstancePerDependency()" : ".SingleInstance()");
                        sb.AppendLine(autofacExportDefinition);

                        Console.WriteLine(autofacExportDefinition);
                    }
                    
                }

            }

I also used Ndepend as a supporting tool to look at visualizations of these dependencies. I had trouble detecting ImportingAttribute on properties (properties are methods in C# known as 'property getters' and 'property setters'), but at least I came up with the following CQLinq statements to look for all types that are decorated with the ExportAttribute (exporting parts) and Ndepend then got nice visualization of something called 'View internal dependency cycles on graph'. As Ndepend or my CQLinq skills lacking could not find the importing attribute decorated on properties (Ndepend does not fully support attribute detection on methods in an easy way yet - detecting attributes on types is easier), I ended up with the CQLinq below to at least list up the exporting classes and launching the graphical tool to look if the parts (classes) with circular dependencies was a hotspot in the source base, i.e. a class used by many other classes. The CQLinq below shows how to generate such a graph for revealing class interdepenencies - quite easy using Ndepend.

let exportingTypes = from type in JustMyCode.Types
where type.IsClass && type.HasAttribute("System.ComponentModel.Composition.ExportAttribute")
&& (type.FullNameLike("SomeAcme"))
let f = Types.ChildMethods().Where(m =< m.IsPropertyGetter)
select type
let typesAttributes = Types
from m in exportingTypes.UsingAny(typesAttributes).ChildMethods().Where(x =< x.IsPropertySetter || x.IsPropertyGetter)
let mAttributes = typesAttributes.Where(t =< m.HasAttribute(t)).ToArray()
where mAttributes .Length < 0
select new { m, mAttributes} 

So there you have some tips around how to migrate from MEF to Autofac and detect cyclic dependencies in your source code. Ndepend will be a good tool to have as a companion to the refactoring job when you want to migrate. I will suggest to first rewrite your application using importing constructors instead of property based imports and then fix up the cyclic dependencies. You can use the Lazy initializer for example. It will delay constructing a part of type T specified to fix up such circular dependencies. Or you could of course refactor the code such that part A and part B that imports eachother instead imports some other part C both.. There are different ways to fix it up. Once you have rewritten the software to use importing constructors and there are no circular dependencies, you can switch to Autofac. I showed you in the unit test OutputAutofacRegistrations how to do that. It outputs ContainerBuilder statements to build up a working Autofac IContainer with same kind of mesh of dependencies as in the MEF based application.

Building Angular apps with source maps and vendor chunk

Here is a quick tip about controlling the generation of source maps and vendor chunks in Angular 8 apps. Sourcemaps are built default in Angular according to the documentation @ https://angular.io/cli/build To be specific, the following command worked for me:

ng build --prod --sourceMap

In addition, the vendor chunk is now baked into the main chunk. To create a separate vendor chunk, run this: Angular 8 will put the vendor chunk into the main for optimizing the js code.

ng build --prod --sourceMap --vendor-chunk=true

In addition, it is recommended to analyze the bundle size with for example Webpack Bundle Analyzer like this: npm install -g webpack-bundle-analyzer Then add the following Npm run scripts to package.json:

    "buildwithstats": "ng b --sourceMap --prod --stats-json",
    "analyze": "webpack-bundle-analyzer --port 9901 dist/stats.json",

Now we have an interactive TreeMap view we can zoom into and see what is taking up space in our bundle!

Searching for 'Angularized values' in MS Sql Server DB

The following query can be used to look for 'Angularized values' in MS Sql Server DB.

DECLARE @sql nvarchar(1024)
DECLARE @column_name varchar(200)
DECLARE @table_name varchar(200)
DECLARE @sp_out varchar(2048)
DECLARE @x int

create table #tmp(sp_out varchar(2048), table_name varchar(255), column_name varchar(2048), id uniqueidentifier)
   
DECLARE tn_cursor CURSOR FOR
SELECT TABLE_NAME
FROM INFORMATION_SCHEMA.tables

open tn_cursor 
FETCH NEXT FROM tn_cursor
INTO @table_name

WHILE @@FETCH_STATUS = 0    
BEGIN   
                print @table_name

                DECLARE cn_cursor CURSOR FOR
                SELECT COLUMN_NAME
                FROM INFORMATION_SCHEMA.columns
                WHERE TABLE_NAME = @table_name

                open cn_cursor 
                FETCH NEXT FROM cn_cursor
                INTO @column_name

                WHILE @@FETCH_STATUS = 0    
                BEGIN   
                                FETCH NEXT FROM cn_cursor
                                INTO @column_name

                                --print @column_name

                                set @sql = 
                                N'insert into #tmp SELECT TOP (1000) ' + @column_name + ', ''' + @table_name + ''', ''' + @column_name + ''', Id' +
                                N' FROM [dbo].[' + @table_name + N']' +
                                N' WHERE ' + @column_name + N' like ''%? % ?%'''
                
                                --print @sql
                                exec sp_executesql @sql
                END

                CLOSE cn_cursor;    
                DEALLOCATE cn_cursor; 

                FETCH NEXT FROM tn_cursor
                INTO @table_name
END

CLOSE tn_cursor;    
DEALLOCATE tn_cursor; 

select * from  #tmp
drop table #tmp

This query will look for all fields in the DB with the contents with pattern '%? % %?' that matches Angularized values. An Angularized value is generated by AngularJs when autogenerated values are made when the contents of a drop down does not match the HTML.