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Singleton

04 Mar

Ensure a class has only one instance and provide a global point of access to it.

UML class diagram

participants

The classes and/or objects participating in this pattern are:

  • Singleton (LoadBalancer)
    • defines an Instance operation that lets clients access its unique instance. Instance is a class operation.
    • responsible for creating and maintaining its own unique instance.

using 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;
}
}
}
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Posted by on March 4, 2010 in C#, Design Pattern

 

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