static void Main(string[] args)
        {
            var list = GetData();

            var query =
                    from u in list
                    where u.IsNormal == "1"
                    group u by u.UserId into g
                    orderby g.Key
                    from u2 in g
                    where u2.Date == g.Min(u3 => u3.Date)
                    select u2;

            foreach (var u in query)
                Console.WriteLine("{0}, {1}, {2}", u.UserId, u.Date, u.Rank);
            

            Console.Read();
        }

        static IEnumerable<User> GetData()
        {
            yield return new User { UserId = "100", Rank = "10", Date = "20101012", IsNormal = "1" };
            yield return new User { UserId = "100", Rank = "20", Date = "20101009", IsNormal = "1" };
            yield return new User { UserId = "200", Rank = "11", Date = "20101011", IsNormal = "1" };
            yield return new User { UserId = "100", Rank = "18", Date = "20101008", IsNormal = "1" };
            yield return new User { UserId = "300", Rank = "2", Date = "20101010", IsNormal = "1" };
            yield return new User { UserId = "300", Rank = "15", Date = "20101013", IsNormal = "1" };
            yield return new User { UserId = "300", Rank = "16", Date = "20101002", IsNormal = "0" };
        }