Singleton design pattern

Microsoft .NET Framework, ASP.NET, Visual C# (CSharp, C Sharp, C-Sharp) Developer Training, Visual Studio

The Singleton design pattern is the most widely known pattern and the simplest of all patterns: The class diagram comprises a single class—the Singleton.

Use the Singleton design pattern when there must be only one instance of a class and it must be accessible to clients via a known access point.

Classic Singleton design pattern

The classic Singleton design pattern is often seen and quoted; however, it has a major flaw—it is not thread safe. Therefore, do not use it. It is shown here; so, it can be shown where its fault lies.

C# example of classic Singleton design pattern (lazy instantiation, thread unsafe)

// WARNING: This code is not thread safe!
 
using System;
 
public sealed class Singleton
{
 
   private static Singleton singleton;
 
   private Singleton() {} // private constructor
 
   public static Singleton GetInstance()
   {
      if (singleton == null)
      {
         singleton = new Singleton();  // lazy
         Console.WriteLine ("Singleton instantiated");
      }
      return singleton;
   }
}
 
class SingletonDemo
{
   static void Main()
   {
      Singleton objectA = Singleton.GetInstance();
      Singleton objectB = Singleton.GetInstance();
      // Singleton objectC = new Singleton();  - won't compile
      if (objectA == objectB)
      {
         Console.WriteLine ("Both refer to the same instance");
      }
   }
}

Notes on classic Singleton design pattern

1. It is not essential to mark members as private; since—unlike the Java language—, the default access is private.
2. It is not essential to make the class sealed; since, Singleton cannot be subclassed already due to its private constructor.
3. One could just as easily use a static property named Instance in lieu of the GetInstance() method.

public static Singleton Instance
{
   get
   {	        
      if (singleton == null)
      {
         singleton = new Singleton();
      }
      return singleton;
   }
}