C# Overriding default interface methods

Question

Is it possible to override default interface methods in another interface?

Assume I have the following code:

public class Program
{
    public static void Main()
    {
        IOriginal origOrig = new Original();
        IOriginal origInh = new Inherited();
        IInherited inherited = new Inherited();
        
        Console.WriteLine($"IOriginal Original: {origOrig.Method()}");
        Console.WriteLine($"IOriginal Inherited: {origInh.Method()}");
        Console.WriteLine($"IInherited Inherited: {inherited.Method()}");
    }
}

public interface IOriginal
{
    string Method() => "original";
}

public interface IInherited : IOriginal
{
    string Method() => "inherited";
}

public class Inherited : IInherited {}
public class Original : IOriginal {}

The output is

IOriginal Original: original
IOriginal Inherited: original
IInherited Inherited: inherited

However, I would like to override the method in IInherited interface, so it produces this:

IOriginal Original: original
IOriginal Inherited: inherited
IInherited Inherited: inherited

In the C# 8 specification proposal , it was proposed by doing this:

interface IA
{
    void M() { WriteLine("IA.M"); }
}
interface IB : IA
{
    override void IA.M() { WriteLine("IB.M"); } // explicitly named
}
interface IC : IA
{
    override void M() { WriteLine("IC.M"); } // implicitly named
}

But in the official release, it is not working?

Compilation error: The modifier 'override' is not valid for this item

Is there any way to achieve this?

Answer 1

Using:

interface IB : IA
{
    void IA.M() { WriteLine("IB.M"); } // explicitly named
}

appears to work .

I can't find any C# docs saying that this is the correct syntax however!