using System;

public class Program

{

    public static void Main()

    {

        var patent = new Patent();

        patent.isDeleted = false;

        CheckDeleted(patent);

        patent.isDeleted = true;

        CheckDeleted(patent);

        patent.isDeleted = null;

        CheckDeleted(patent);       

    }

    public static void CheckDeleted(Patent patent)

    {

        if (patent.isDeleted == true)

            Console.WriteLine("TRUE");

        if (patent.isDeleted == false)

            Console.WriteLine("FALSE");

        if (patent.isDeleted == null)

            Console.WriteLine("NULL");

    }   

}

​

public class Patent

{

    public bool? isDeleted { get; set; }

}