Why do I need to cast 'this' to an interface with a default implementation in C# 8.0 when I call it in the class derived form that interface?

user_1777224 0
Viewed 788

I have this Simple Console program in .NET Core 3.1 with C# 8:

using System;

namespace ConsoleApp34
{

    public interface ITest
    {
        public void test()
        {
            Console.WriteLine("Bye World!");

        }
    }

    public class Test : ITest
    {
        public void CallDefault()
        {
            ((ITest)(this)).test();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            var t = new Test();
            t.CallDefault();

        }
    }
}

I don't understand why the cast is necessary in the line ((ITest)(this)).test();

Test is directly derived from ITest, so, by definition, 'this' IS ITest

Thank you.

default-interface-member c#-8.0 .net-core c#
2 Answers

Default interface implementations work similarly to explicit implementations: they can only be called through the interface type, not the implementing type.

To understand why this is so, imagine Test implemented 2 interfaces with the same method signature; which would be used if there was no cast?

public interface ITest2
{
    public void test()
    {
        Console.WriteLine("Hello World!");
    }
}

public class Test : ITest, ITest2
{
    public void CallDefault()
    {
        test(); // Do we use ITest.test() or ITest2.test()?
    }
}
user_81263620

This behavior is documented here.

Beginning with C# 8.0, you can define an implementation for members declared in an interface. If a class inherits a method implementation from an interface, that method is only accessible through a reference of the interface type. The inherited member doesn't appear as part of the public interface. The following sample defines a default implementation for an interface method:

public interface IControl
{
    void Paint() => Console.WriteLine("Default Paint method");
}
public class SampleClass : IControl
{
    // Paint() is inherited from IControl.
}

The following sample invokes the default implementation:

var sample = new SampleClass();
//sample.Paint();// "Paint" isn't accessible.
var control = sample as IControl;
control.Paint();

Any class that implements the IControl interface can override the default Paint method, either as a public method, or as an explicit interface implementation.

user_11851360
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