Unexpected Interface in Class base list

[bernd5]

Steps to reproduce

Compile:

new F().Bar();
new SubF().Bar();


class F : F.IFoo
{
    protected interface IFoo
    {
        void Foo();
    }

    void IFoo.Foo()
    {
        System.Console.WriteLine("F");
    }

    public void Bar()
    {
        ((IFoo)this).Foo();
    }
}

class SubF : F, SubF.ISubFoo
{
    protected interface ISubFoo : F.IFoo { }

    void F.IFoo.Foo()
    {
        System.Console.WriteLine("SubF");
    }
}

Decompiled CSharp:

using System;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Security;
using System.Security.Permissions;

[assembly: CompilationRelaxations(8)]
[assembly: RuntimeCompatibility(WrapNonExceptionThrows = true)]
[assembly: Debuggable(DebuggableAttribute.DebuggingModes.IgnoreSymbolStoreSequencePoints)]
[assembly: SecurityPermission(SecurityAction.RequestMinimum, SkipVerification = true)]
[assembly: AssemblyVersion("0.0.0.0")]
[module: UnverifiableCode]
[module: RefSafetyRules(11)]

internal class F : F.IFoo
{
    protected interface IFoo
    {
        void Foo();
    }

    void IFoo.Foo()
    {
        Console.WriteLine("F");
    }

    public void Bar()
    {
        ((IFoo)this).Foo();
    }
}

internal class SubF : F, SubF.ISubFoo, F.IFoo //<-- F.IFoo is not implemented directly
{
    protected interface ISubFoo : IFoo
    {
    }

    void IFoo.Foo()
    {
        Console.WriteLine("SubF");
    }
}

The interface list of SubF contains an unexpected F.IFoo. This is invalid CSharp - it would not compile.

[dgrunwald]

Huh, so SubF's base class list has visibility to the protected SubF.ISubFoo, but not to the protected SubF.IFoo/F.IFoo.
That is surprising.
The old (pre-roslyn) csc also rejects your original code.

I think we can only try to detect this situation and suppress the base list entry.