gistfile1.txt

using System;

namespace ConsoleApplication1
{
    interface ITest
    {
        void Method();
    }

    class Parent : ITest
    {
        void ITest.Method()
        {
            Console.WriteLine("Parent");
        }
    }

    class Child : Parent, ITest
    {
        void ITest.Method()
        {
            Console.WriteLine("Child");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var c = new Child();
            Parent p = c;
            object o = p;
            ((ITest)c).Method();  // Child
            ((ITest)p).Method();  // Child
            ((ITest)o).Method();  // Child
        }
    }
}

OtherTest.cs

using System;

namespace ConsoleApplication1
{
    interface ITest
    {
        void Method();
    }

    class Parent : ITest
    {
        void ITest.Method() { Console.WriteLine("Called Parent.Method. Current type {0}", GetType());}
        public void ParentMethodAccessor()
        {
            ((ITest)this).Method();
        }
    }

    class Child : Parent, ITest
    {
        void ITest.Method() { Console.WriteLine("Called Child.Method. Current type {0}", GetType()); }
        public void ChildMethodAccessor()
        {
            ((ITest)this).Method();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Parent parent = new Parent();
            Child child = new Child();
            parent.ParentMethodAccessor();
            child.ChildMethodAccessor();
            ITest parentTest = parent;
            ITest childTest = child;
            parentTest.Method();
            childTest.Method();
            Parent childAsParent = child;
            childAsParent.ParentMethodAccessor();
            ITest childAsParentAsTest = childAsParent;
            childAsParentAsTest.Method();
            Console.ReadLine();
        }
    }
}