using System;

public class Program

{   

    public static void Main()

    {

        DateTime? a = null;

        DateTime? b = DateTime.Now;

        compare(a.GetValueOrDefault(),b);

        compare(a,b);

    }

    private static void compare(DateTime? a, DateTime? b)

    {

        if (a < b)

            Console.WriteLine("'{0}' is bigger than '{1}'.", b, a);

        else if (a > b)

            Console.WriteLine("'{0}' is bigger than '{1}'.", a, b);

        else if ( a == b )

            Console.WriteLine("Both '{0}' and '{1}' are equal.", a, b);

        else

            Console.WriteLine("'{0}' and '{1}' are not equal.", a, b);

​

        Console.WriteLine("--------------------------------------");

    }

}