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Null unification

using System; using System.Collections.Generic; using System.Reflection; using System.Runtime.InteropServices; public class MainClass {    public static void Main(){         int? x1 = null;         int? x2 = new int?();         int? y1 = 55;         int? y2 = new int?(55);         Console.WriteLine("{0}...{1}...{2}...{3}", x1, x2, y1, y2);         Console.WriteLine("x1 == null? {0}", x1 == null);         object x1o = x1;          Console.WriteLine("x1o == null? {0}", x1o == null);         Console.WriteLine("{0}...{1}", x1.GetType(), x1o.GetType());    } } ......55...55 x1 == null? True x1o == null? True Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an ob ject. at MainClass.Main()