Example 1:

using System;

namespace DoFactory.GangOfFour.Singleton.Structural
{
  
  // MainApp test application 

  class MainApp
  {
    
    static void Main()
    {
      // Constructor is protected -- cannot use new 
      Singleton s1 = Singleton.Instance();
      Singleton s2 = Singleton.Instance();

      if (s1 == s2)
      {
        Console.WriteLine("Objects are the same instance");
      }

      // Wait for user 
      Console.Read();
    }
  }

  // "Singleton" 

  class Singleton
  {
    private static Singleton instance;

    // Note: Constructor is 'protected' 
    protected Singleton() 
    {
    }

    public static Singleton Instance()
    {
      // Use 'Lazy initialization' 
      if (instance == null)
      {
        instance = new Singleton();
      }

      return instance;
    }
  }
}
							


Example2:

/// 

/// Class implements singleton pattern.
/// 

public class Singleton
{
        // Private constructor to avoid other instantiation
        // This must be present otherwise the compiler provide 
        // a default public constructor
        private Singleton()
        {
        }
 
        /// 

        /// Return an instance of 
        /// 

        public static Singleton Instance
        {
            get
            {
                /// An instance of Singleton wont be created until the very first 
                /// call to the sealed class. This a CLR optimization that ensure that
                /// we have properly lazy-loading singleton. 
                return SingletonCreator.CreatorInstance;
            }
        }
 
        /// 

        /// Sealed class to avoid any heritage from this helper class
        /// 

        private sealed class SingletonCreator
        {
          // Retrieve a single instance of a Singleton
          private static readonly Singleton _instance = new Singleton();
 
          /// 

          /// Return an instance of the class 
          /// 

          public static Singleton CreatorInstance
          {
            get { return _instance; }
          }
        }
}