using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public class Foo {

        public int Id {get; set; }

        public string Data {get; set;}

    }

    public static void Main()

    {

        List<Foo> x = new List<Foo> { new Foo {Id = 1, Data = "1-one"}, new Foo {Id = 1, Data = "1-two"}, new Foo {Id = 2, Data = "2-one"} };

        foreach (var g in x.GroupBy(i => i.Id)) {

            g.ToList().Dump();

        }

    }

}