using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        IEnumerable<int> numbers = new List<int> {1,2,3,4,5,6};

        var result = numbers.GenericExt<IEnumerable<int>,String>();

    }

}

​

static class Extensions

{

    public static TU GenericExt<T, TU>(this T obj)

    {

         TU ret = default(TU);

         // do some stuff with obj

         return ret;

    }

​

}