Test.cs

using System;

public interface IFoo
{
    void M(int x = 0);
}

public class Foo : IFoo
{
    public void M(int x = 10)
    {
        Console.WriteLine(x);
    }
}

class Test
{
    static void Main()
    {
        IFoo foo = new Foo();
        foo.M();
    }
}