C# 9 covariant return types: Still can get System.TypeLoadException when using generics

[ASolomatin]

Description

This looks like dotnet/core#5319 but with generics.

The C # 9.0 overridden virtual function works fine when the return type is a covariant derived type, continues to work if the return type is a covariant derived type that contains the overridden function. But System.TypeLoadException is thrown at runtime if you add some generics. As before, everything builds without errors and warnings.

Code to reproduce

using System;

var c = new AChild();

Console.WriteLine("Hello World!");

public abstract class A<T>
{
    public abstract B<T> GetB();
}

public class AChild : A<string>
{
    public override BChild GetB() => null;
}

public class B<T>
{
}

public class BChild : B<string>
{
}

Error:

$ dotnet run
Unhandled exception. System.TypeLoadException: Return type in method 'AChild.GetB()' on type 'AChild' from assembly 'N5, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not compatible with base type method 'A`1[System.__Canon].GetB()'.
$ dotnet --version
5.0.100

Versions

5.0.100-rc.2.20479.15
5.0.100

Originally posted by @ASolomatin in dotnet/core#5319 (comment)

[MrKWatkins]

This also happens in 5.0.100 if you just return T directly, rather than a type that uses T, e.g.

using System;

_ = new CovariantReturn();

public abstract class Base<T>
{
   public virtual T Get() => throw new NotImplementedException();
}

public sealed class CovariantReturn : Base<object>
{
   public override string Get() => throw new NotImplementedException();
}

[MichalStrehovsky]

Sounds like #45037/#45082

Cc @janvorli

[janvorli]

I've verified this issue is fixed by #45275, it is a dup of #45037

[adamjones1]

Hi, in the meantime before this fix is released, I am trying to continue developing my app on machines with 5.0.101 installed (I didn't experience the error on 5.0.100 for some reason, it only appeared when 5.0.101 was installed). Is there any way to force usage of 5.0.100 for running it with a global.json? When I attempt this, dotnet --version gives me 5.0.100 but when the app actually launches unit tests with dotnet test it still gives me the error as if it were ignoring the global.json.

[janvorli]

Would setting the runtime version property in the csproj file to 5.0.100 work as a workaround?

<RuntimeFrameworkVersion>5.0.100</RuntimeFrameworkVersion>

[adamjones1]

Trying this I get "Unable to find package Microsoft.NETCore.App.Host.win-x64 with version (= 5.0.100)" (seems only 5.0.1 exists at https://www.nuget.org/packages/Microsoft.NETCore.App.Host.win-x64/?). Using 5.0.1 instead the issue is still present I assume as this still instructs it to use the latest patch version.

[janvorli]

I am sorry for the confusion, I meant 5.0.0.

[adamjones1]

Ah ok, using 5.0.0 unfortunately the TypeLoadException is still present. My full csproj is:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
    <Nullable>enable</Nullable>
    <IsPackable>false</IsPackable>
    <RuntimeFrameworkVersion>5.0.0</RuntimeFrameworkVersion>
  </PropertyGroup>
  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
    <ProjectReference Include="..\..\src\MyProject\MyProject.csproj" />
  </ItemGroup>
  <Import Project="..\..\.paket\Paket.Restore.targets" />
</Project>

[ASolomatin]

Still can reproduce with small code modifications:

using System;

var c = new AChild<string>();

Console.WriteLine("Hello World!");

public abstract class A<T>
{
    public abstract B<T> GetB();
}

public class AChild<T> : A<T>
{
    public override BChild<T> GetB() => null;
}

public class B<T>
{
}

public class BChild<T> : B<T>
{
}

[janvorli]

@ASolomatin what version of runtime have you reproed it with? The latest fix is not in any released .NET 5 versions, it will be part of 5.0.2.

[ASolomatin]

Sorry. ) I thought, it's already released because the original issue stops to reproduce on 5.0.1