using System;

namespace ConsoleApplication1

{

    public class Program

    {

        public static void Main(string[] args)

        {

            const string format = "yyyy-MM-ddTHH:mm:ss.fff";

            var trimTime = new char[] { '0', '.' };

            var a = new DateTime(2016, 3, 3, 12, 11, 10, 123);

            var b = new DateTime(2016, 3, 3, 12, 11, 10, 120);

            var c = new DateTime(2016, 3, 3, 12, 11, 10, 100);

            var d = new DateTime(2016, 3, 3, 12, 11, 10, 000);

            Console.WriteLine("a {0} expected 2016-03-03T12:11:10.123", a.ToString(format).TrimEnd(trimTime));

            Console.WriteLine("b {0} expected 2016-03-03T12:11:10.12", b.ToString(format).TrimEnd(trimTime));

            Console.WriteLine("c {0} expected 2016-03-03T12:11:10.1", c.ToString(format).TrimEnd(trimTime));

            Console.WriteLine("d {0} expected 2016-03-03T12:11:10", d.ToString(format).TrimEnd(trimTime));

        }

    }

}

​

​