How to extend IEnumable<T> ?

Here is a example:

 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace LamdaExpressionTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var list = new List<Product>
            {
                new Product{
                    ProductID = 1,
                    ProductName = "aaaa",
                    Description = "this is aaaa"
                },
                new Product{
                    ProductID = 2,
                    ProductName = "bbbb",
                    Description = "this is bbbb"
                },
                new Product{
                    ProductID = 3,
                    ProductName = "cccc",
                    Description = "this is cccc"
                },
                new Product{
                    ProductID = 4,
                    ProductName = "dddd",
                    Description = "this is dddd"
                },
                new Product{
                    ProductID = 5,
                    ProductName = "eeee",
                    Description = "this is eeee"
                },
                new Product{
                    ProductID = 6,
                    ProductName = "ffff",
                    Description = "this is ffff"
                },
            };

            Func<Product, bool> fun = (Product product) =>
            {
                return product.ProductID % 2 == 0;
            };
            var list2 = list.Where(fun).ToList();
            var list3 = list.FilterByHashCode().ToList();

            list.ForEach(item => Console.WriteLine(item.ProductName));
        }
    }

    public class Product
    {
        public int ProductID { set; get; }
        public string ProductName { set; get; }
        public string Description { set; get; }
    }

    public static class IEnumableExtension
    {
        public static IEnumerable<T> FilterByHashCode<T>(this IEnumerable<T> list)
        {
            if (list == null || list.Count() < 1)
            {
                yield break;
            }

            foreach (var item in list)
            {
                if (item.GetHashCode() % 2 == 0)
                {
                    yield return item;
                }
            }
        }

        public static void Foreach<T>(this IEnumerable<T> colection, Action<T> action)
        {
            if(colection == null || colection.Count() < 1)
                return;

            foreach(var item in colection)
            {
                action(item);
            }
        }
    }

}