public class DemoEntity
    {
        public int Date { get; set; }

        public string Class { get; set; }

        public override string ToString()
        {
            return String.Format("{0}-{1}", Date, Class);
        }
    }


    class Program
    {
        static void Main(string[] args)
        {
            List<DemoEntity> myList = new List<DemoEntity>();
            myList.Add(new DemoEntity() { Date = 1, Class = "C1" });
            myList.Add(new DemoEntity() { Date = 1, Class = "C2" });
            myList.Add(new DemoEntity() { Date = 1, Class = "C1" });
            myList.Add(new DemoEntity() { Date = 2, Class = "C1" });
            myList.Add(new DemoEntity() { Date = 2, Class = "C2" });
            myList.Add(new DemoEntity() { Date = 2, Class = "C3" });
            myList.Add(new DemoEntity() { Date = 2, Class = "C2" });

            var result = myList.GroupBy(g => new { g.Class, g.Date });
            result.ToList().ForEach(i => i.ToList().ForEach(a => Console.WriteLine(a.ToString())));
            Console.ReadKey();
        }