using System.Linq;

using System.Collections;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var w1 = FromInMemoryCollection();

        var w2 = FromYielding();

        var w3 = FromLinqing();

        var w4 = FromCustom();

    }

​

    private static IEnumerable<int> FromInMemoryCollection()

    {

        return new[]{0, 1, 2, 3};

    }

​

    private static IEnumerable<int> FromYielding()

    {

        yield return 0;

        yield return 1;

        yield return 2;

        yield return 3;

    }

​

    private static IEnumerable<int> FromLinqing()

    {

        return FromYielding().Where(n => n % 2 == 0);

    }

​

    private static IEnumerable<int> FromCustom()

    {

        return new CustomEnumerable();

    }

​

    private class CustomEnumerable : IEnumerable<int>

    {

        private readonly IEnumerable<int> inner;

        public CustomEnumerable()

        {

            inner = new [] { 0, 1, 2, 3 };

        }

        public IEnumerator<int> GetEnumerator()

        {

            return inner.GetEnumerator();

        }

        IEnumerator IEnumerable.GetEnumerator()

        {

            return this.GetEnumerator();

        }

    }

}