sing System;
 
namespace DoFactory.GangOfFour.Singleton.Structural
{
  /// <summary>
  /// MainApp startup class for Structural
  /// Singleton Design Pattern.
  /// </summary>
  class MainApp
  {
    /// <summary>
    /// Entry point into console application.
    /// </summary>
    static void Main()
    {
      // Constructor is protected -- cannot use new
      Singleton s1 = Singleton.Instance();
      Singleton s2 = Singleton.Instance();
 
      // Test for same instance
      if (s1 == s2)
      {
        Console.WriteLine("Objects are the same instance");
      }
 
      // Wait for user
      Console.ReadKey();
    }
  }
 
  /// <summary>
  /// The 'Singleton' class
  /// </summary>
  class Singleton
  {
    private static Singleton _instance;
 
    // Constructor is 'protected'
    protected Singleton()
    {
    }
 
    public static Singleton Instance()
    {
      // Uses lazy initialization.
      // Note: this is not thread safe.
      if (_instance == null)
      {
        _instance = new Singleton();
      }
 
      return _instance;
    }
  }
}

Output
Objects are the same instance