Strange behaviour on generics

[KeyserDSoze]

I have this situation
a class

 public class MyDocument : IConfigurableDocument
  {
      public string PrimaryKey { get; set; }
      public string SecondaryKey { get; set; }
      public MiniSample Mini { get; set; }
      public DateTime Timestamp { get; set; }

      public RystemDocumentServiceProvider Configure(string callerName)
      {
          return this.StartConfiguration()
              .WithAzure()
              .WithTableStorage(new Azure.Integration.Storage.TableStorageConfiguration(callerName))
              .WithPrimaryKey(x => x.PrimaryKey)
              .WithSecondaryKey(x => x.SecondaryKey)
              .WithTimestamp(x => x.Timestamp)
              .ConfigureAfterBuild();
      }
  }

a class that extends this class

public class MyRealDocument : MyDocument
{
}

In the method Configure() there is an extension method StartConfiguration()

public static class ConfigurableDocumentExtensions
{
    public static RystemDocumentServiceProvider<T> StartConfiguration<T>(this T configurableDocument)
        where T : IConfigurableDocument
        => RystemDocumentServiceProvider.Configure(configurableDocument);
}

And in the extension method I'm using a static method that creates an instance of RystemDocumentServiceProvider

public sealed class RystemDocumentServiceProvider
  {
      private RystemDocumentServiceProvider() { }
      internal static readonly RystemDocumentServiceProvider Instance = new();
      public static RystemDocumentServiceProvider<T> Configure<T>(T entity) 
          => new(ServiceLocatorAtRuntime.PrepareToAddNewService());
  }

Ok, now, when I call the Configure() method in this way.

new MyRealDocument().Configure()

I supposed to have a RystemDocumentServiceProvider as RystemDocumentServiceProvider, when code was running I found instead that that object was a RystemDocumentServiceProvider even though if I put a breakpoint in

public static RystemDocumentServiceProvider<T> Configure<T>(T entity) 
            => new(ServiceLocatorAtRuntime.PrepareToAddNewService());

and I saw that my T entity was a MyRealDocument and not a MyDocument.

Why do I receive that object as MyDocument and not as MyRealDocument?
Is it a normal behaviour or is it strange for something?

[svick]

I'd say this is the expected behavior. You call this.StartConfiguration() inside MyDocument, so that call is resolved as ConfigurableDocumentExtensions.StartConfiguration<MyDocument>(this). It doesn't matter here that the runtime type of this is actually MyRealDocument, because its compile-time type, and thus also the T in StartConfiguration, is still going to be MyDocument.

[KeyserDSoze]

ok I thought the generic was generated at runtime. Then, it's the correct behaviour. And so, C# creates during compiling all the possible classes to fit. Good news for me. How can I solve my problem, exists a best practice or a work-around?

I tried to use reflection (I already knew that the returned type would have been the same T) but I tried.

public static RystemDocumentServiceProvider<T> Configure<T>(T entity)
{
    var type = typeof(RystemDocumentServiceProvider<>);
    var genericType = type.MakeGenericType(entity.GetType());
    var constructor = genericType.GetConstructor(new Type[1] { typeof(IServiceCollection) });
    var instance = constructor.Invoke(new object[1] { ServiceLocatorAtRuntime.PrepareToAddNewService() });
    return (RystemDocumentServiceProvider<T>)instance;
}

[svick]

@KeyserDSoze You don't say what is the problem with the reflection code, but I think it won't work, since you're casting an instance of RystemDocumentServiceProvider<MyRealDocument> to RystemDocumentServiceProvider<MyDocument>, which are two incompatible types. One solution to that would be to create and use a covariant interface IRystemDocumentServiceProvider<out T>, if that makes sense for your code.

[KeyserDSoze]

no no it works fine, but it returns obviously RystemDocumentServiceProvider<MyDocument> but it works without any exception. I don't need to create a covariant interface. In a few words, I need to return RystemDocumentServiceProvider<MyRealDocument> in any way passing for a method in MyDocument. Is there any chance to do it somehow?

[Joe4evr]

No, only interfaces and delegates can have co-/contravariant type parameters.

[KeyserDSoze]

Sorry @svick @Joe4evr I realized what you said only now. Yes you are right, to work with it I used an interface, it's the only way to run that code, but the point is not that, the point is: "Is there any chance to have IRystemDocumentServiceProvider<MyRealDocument> and not IRystemDocumentServiceProvider<MyDocument>?"