NSubstitute/Castle proxy fail to create substitute that returns a derived class from abstract that satisfies interface implicitly

[siblount]

Describe the bug
NSubstitute/Castle proxy fails to create a substitute for a concrete class that derives from an abstract class and virtual members.

To Reproduce
Here's an example of what I am talking about:
Setup classes

namespace TestNSubstituteBS
{
    public abstract class AbstractIOContext
    {
        public abstract IDirectoryInfo CreateDirectoryInfo();
    }
}

namespace TestNSubstituteBS
{
    public class ConcreteIOContext : AbstractIOContext
    {
        public override XDirectoryInfo CreateDirectoryInfo() => new XDirectoryInfo();
    }
}

namespace TestNSubstituteBS
{
    public class XDirectoryInfo : IDirectoryInfo { }
}

namespace TestNSubstituteBS
{
    public interface IDirectoryInfo { }
}

Test code:

namespace TestNSubstituteBS.Tests
{
    [TestClass()]
    public class AbstractFactoryTests
    {
        [TestMethod]
        public void CreateTest()
        {
            var a = Substitute.For<ConcreteIOContext>(); // <--------------- fails here
            a.CreateDirectoryInfo().Returns(new XDirectoryInfo());
        }
    }
}

Running the above returns this error message...

Test method TestNSubstituteBS.Tests.AbstractFactoryTests.CreateTest threw exception: 
System.TypeLoadException: Return type in method 'Castle.Proxies.ConcreteIOContextProxy.CreateDirectoryInfo()' on type 'Castle.Proxies.ConcreteIOContextProxy' from assembly 'DynamicProxyGenAssembly2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is not compatible with base type method 'TestNSubstituteBS.ConcreteIOContext.CreateDirectoryInfo()'.

and stack trace...

  Stack Trace: 
TypeBuilder.CreateTypeNoLock()
TypeBuilder.CreateTypeInfo()
AbstractTypeEmitter.BuildType()
BaseClassProxyGenerator.GenerateType(String name, INamingScope namingScope)
<>c__DisplayClass13_0.<GetProxyType>b__0(CacheKey cacheKey)
SynchronizedDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
BaseProxyGenerator.GetProxyType()
DefaultProxyBuilder.CreateClassProxyType(Type classToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options)
ProxyGenerator.CreateClassProxyType(Type classToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options)
ProxyGenerator.CreateClassProxy(Type classToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options, Object[] constructorArguments, IInterceptor[] interceptors)
CastleDynamicProxyFactory.CreateProxyUsingCastleProxyGenerator(Type typeToProxy, Type[] additionalInterfaces, Object[] constructorArguments, IInterceptor[] interceptors, ProxyGenerationOptions proxyGenerationOptions)
CastleDynamicProxyFactory.GenerateTypeProxy(ICallRouter callRouter, Type typeToProxy, Type[] additionalInterfaces, Object[] constructorArguments)
CastleDynamicProxyFactory.GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[] additionalInterfaces, Object[] constructorArguments)
SubstituteFactory.Create(Type[] typesToProxy, Object[] constructorArguments, Boolean callBaseByDefault)
SubstituteFactory.Create(Type[] typesToProxy, Object[] constructorArguments)
Substitute.For(Type[] typesToProxy, Object[] constructorArguments)
Substitute.For[T](Object[] constructorArguments)
AbstractFactoryTests.CreateTest() line 18

Expected behaviour
Preferibly, that it works. Otherwise, an error message about how much of an idiot I am.
Environment:

• NSubstitute version: 5.0.0
• NSubstitute.Analyzers version: 1.0.16
• Platform: .NET 6.0 Windows

Additional context

This occurs for Substitute.For<>() and Substitute.ForPartsOf<>().

[siblount]

Also note, using Moq, there is no issue with this:

public void CreateTest()
{
    var a = new Mock<ConcreteIOContext>();
    a.Setup(x => x.CreateDirectoryInfo()).Returns(new XDirectoryInfo());
}

[siblount]

I lied. It seems to be an issue with ConcreteIOContext.CreateDirectoryInfo() with the return type of XDirectoryInfo instead of IDirectoryInfo, which is what the AbstractIOContext implies. In other words, replacing the code above with this:

namespace TestNSubstituteBS
{
    public class ConcreteIOContext : AbstractIOContext
    {
        // public override XDirectoryInfo CreateDirectoryInfo() => new XDirectoryInfo(); <-- does not work
        public override IDirectoryInfo CreateDirectoryInfo() => new XDirectoryInfo(); // <--- this does work
    }
}

The code above now makes it work without any issue.

[siblount]

It seems that from castleproject/Core#659 (comment) that the issue has been resolved with a later version of Castle.Core supporting C# 9 covariants (which would hopefully be my issue).

So maybe update Castle.Core to a version greater than 5.1.0?

[dtchepak]

Thanks @siblount . I've updated Castle.Core in the MR readying next release: #731

There's an appveyor build if you want to try it out: https://ci.appveyor.com/project/NSubstitute/nsubstitute/builds/47889349/artifacts

[siblount]

Tried out the CI build and I'm glad to say that the issue no longer persists. Thanks @dtchepak, closing this.