What are Singleton classes in C sharp?

eg:using System; 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; } } } read less

What I fail to understand is why do people not take a look at enhancements in a Programming Language before implementing the desired solution! A Singleton, as rightly explained by most of the people here, addresses the need for an Object of a Class to be created only once during the run of the application. For example, the System.Console Class provides access to the Standard Input, Output and Error Device on the system. There can be only one such device per system - hence the need for it to be a Singleton. There are several such classes within the .NET Framework which are architected as Singleton classes. Another example is the ABOUT window of a typical desktop application – there should be only one About window open. Or for example, the File Open or File Print dialog window – there should be only one of these dialog boxes open, across all running instances of the application. So, the question is how does one create a Singleton class. It is one of the firstly taught/learnt Creational Design Pattern. As rightly stated by some of the contributors, the standard approach is to create a private constructor, and enforce an instance of the type is exposed to the outside world by a complex object creation method (following the Factory Method Pattern). But then, remember you are now working in .NET. The people who designed the Framework , while defining the standards that each and every language have to follow – while defining the Common Language Specification – have addressed this fundamental need in an extremely beautiful way. All you have to now do is define the class as a Static Class (which enforces that all members are now Static – meaning only one copy can exist) – and define whatever you need as Public members within the class. For example, System.Console class has been defined as: public static class System.Console { … public static void Beep() { … } public static void WriteLine(string) { … } …. } A static class is just another class – except that it enforces the policy that you cannot instantiate it at all. The class is loaded by the CLR’s Class Loader, the first time it is required, and remains in memory, until the application is unloaded. It cannot be disposed – and hence should be used only when absolutely necessitated. It is because of this very property that a Static Class is used to help define Extension Methods, since only one copy of the extended code can exist for all the objects of the extended type. read less

The singleton pattern is one of the best-known patterns in software engineering. Essentially, a singleton is a class which only allows a single instance of itself to be created, and usually gives simple access to that instance. Most commonly, singletons don't allow any parameters to be specified when creating the instance - as otherwise a second request for an instance but with a different parameter could be problematic! (If the same instance should be accessed for all requests with the same parameter, the factory pattern is more appropriate.) using System; public class Singleton { private static Singleton instance; private Singleton() {} public static Singleton Instance { get { if (instance == null) { instance = new Singleton(); } return instance; } } } read less