using System;

public class Program

{

    public static void Main()

    {

        Guid? nullGuid = null;

        Guid? emptyGuid = Guid.Empty;

        Guid? filledGuid = Guid.NewGuid();

        if (nullGuid.IsNullOrDefault()){

            Console.WriteLine("It's null or default!");

        }

        if (emptyGuid.IsNullOrDefault()){

            Console.WriteLine("It's null or default!");

        }

        if (filledGuid.IsNullOrDefault()){

            Console.WriteLine("It's null or default!");

        }

        Console.WriteLine("Hello World");

    }

}

​

public static class NullableExtensions

{

    public static bool IsNullOrDefault<T>(this T? self) where T : struct

    {

        return !self.HasValue || self.Value.Equals(default(T));

    }

}