This demo shows how to build a class that implements one or more interfaces and make use of an existing implementation through delegates and composition.

DefaultImplementationWithDelegates.cs

using System;

namespace DefaultImplementationWithDelegates
{
    interface ISomeInterface
    {
        string Foo();
    }

    interface IAnotherInterface
    {
        string Bar();
    }

    class DefaultAnotherImplementation : IAnotherInterface
    {
        public string Bar() => "bar";
    }

    class SomeClass : ISomeInterface, IAnotherInterface
    {
        public delegate string BarDelegate();
        BarDelegate BarDelegateMethod;
        public SomeClass(BarDelegate barDelegate)
        {
            BarDelegateMethod = barDelegate;
        }

        public string Foo() => "foo";

        public string Bar() => BarDelegateMethod();
    }

    class Program
    {
        static void Main(string[] args)
        {
            var some = new SomeClass(new DefaultAnotherImplementation().Bar);

            Console.WriteLine(some.Foo());  // >> foo
            Console.WriteLine(some.Bar());  // >> bar

            Console.ReadLine();
        }
    }
}