Singleton Pattern

public class User
{

     private int _userId = 1;
     private static User _instance = new User();

     static User()
     {
     }
     private User()
     {
     }
     public int UserId
     {
         get
         {
             return _userId ;
         }
         set
         {
             _userId = value;
         }
     }

     public static User Instance
     {
         get
         {
             return _instance;
         }
     }
}

[Test]
 public void TestSingleton()
 {
     User sngle1 = User.Instance;
     sngle1.UserId = 1;
     User sngle2 = User.Instance;
     sngle2.UserId = 2;
     Assert.AreEqual(sngle2.UserId, sngle1.UserId);
 }

More Info:

Everything you need to know about C# singleton.

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