ReflectionTypeLoadException exception in Asp.Net Core in-memory TestServer

Question

I create a IWebHostBuilder like so:

public IWebHostBuilder GetWebHostBuilder()
{
     return new WebHostBuilder().UseContentRoot(_contentRoot)
                                       .ConfigureServices(InitializeServiceCollection)
                                       .UseEnvironment(_environment)
                                       .UseConfiguration(GetConfiguration())
                                       .UseStartup(typeof(TStartup));
}

Here, the InitializeServiceCollection is implemented like so:

private void InitializeServiceCollection(IServiceCollection services)
{
     var manager = new ApplicationPartManager();

     manager.ApplicationParts.Add(new AssemblyPart(_assembly));
     manager.FeatureProviders.Add(new ControllerFeatureProvider());
     manager.FeatureProviders.Add(new ViewComponentFeatureProvider());

     services.AddSingleton(manager);
}

Then I create the TestServer like so:

var myTestServer = new TestServer(GetWebHostBuilder());

Here I get the exception (full exception below). It gets thrown at services.AddAutoMapper(); method call in the system under test. However, when I run the system under test on it's own and test it manually with Postman, it's working fine and the object mapping with Automapper is working well too. It just raises the exception in the integration test.

Full exception:

System.AggregateException : One or more errors occurred. (Unable to load one or more of the requested types.
Could not load type 'Microsoft.EntityFrameworkCore.Metadata.Internal.RelationalFullAnnotationNames' from assembly 'Microsoft.EntityFrameworkCore.Relational, Version=2.2.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'.) (The following constructor parameters did not have matching fixture data: TestFixture fixture)
---- System.Reflection.ReflectionTypeLoadException : Unable to load one or more of the requested types.
Could not load type 'Microsoft.EntityFrameworkCore.Metadata.Internal.RelationalFullAnnotationNames' from assembly 'Microsoft.EntityFrameworkCore.Relational, Version=2.2.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'.
---- The following constructor parameters did not have matching fixture data: TestFixture fixture

----- Inner Stack Trace #1 (System.Reflection.ReflectionTypeLoadException) -----
   at System.Reflection.RuntimeModule.GetTypes(RuntimeModule module)
   at System.Reflection.RuntimeAssembly.get_DefinedTypes()
   at System.Linq.Enumerable.SelectManySingleSelectorIterator`2.ToArray()
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at AutoMapper.ServiceCollectionExtensions.AddAutoMapperClasses(IServiceCollection services, Action`2 configAction, IEnumerable`1 assembliesToScan) in xxx\automapper-extensions-microsoft-dependencyinjectio\src\AutoMapper.Extensions.Microsoft.DependencyInjection\ServiceCollectionExtensions.cs:line 72
   at xxx.Startup.ConfigureServices(IServiceCollection services) in xxx\Startup.cs:line 35
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Hosting.ConventionBasedStartup.ConfigureServices(IServiceCollection services)
   at Microsoft.AspNetCore.Hosting.Internal.WebHost.EnsureApplicationServices()
   at Microsoft.AspNetCore.Hosting.Internal.WebHost.Initialize()
   at Microsoft.AspNetCore.Hosting.WebHostBuilder.Build()
   at Microsoft.AspNetCore.TestHost.TestServer..ctor(IWebHostBuilder builder, IFeatureCollection featureCollection)
   at xxx.Tests.TestFixture.InitializeServer() in xxx.Tests\TestFixture.cs:line 67
   at xxx.Tests.TestFixture..ctor() in xxx.Tests\TestFixture.cs:line 31
----- Inner Stack Trace #2 (Xunit.Sdk.TestClassException) -----

Answer 1

So, I managed to resolve the issue by replacing services.AddAutoMapper() with services.AddAutoMapper(Type[] types) overload. Not sure what is wrong with the original method, but in order to avoid that reflective call on the assemblies, I used this overload and it's working now.