Implementing the Singleton Pattern in C#

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.

All these implementations share four common characteristics, however:

• A single constructor, which is private and parameterless. This prevents other classes from instantiating it (which would be a violation of the pattern). Note that it also prevents subclassing - if a singleton can be subclassed once, it can be subclassed twice, and if each of those subclasses can create an instance, the pattern is violated. The factory pattern can be used if you need a single instance of a base type, but the exact type isn't known until runtime.
• The class is sealed. This is unnecessary, strictly speaking, due to the above point, but may help the JIT to optimise things more.
• A static variable which holds a reference to the single created instance, if any.
• A public static means of getting the reference to the single created instance, creating one if necessary.

public sealed class Singleton

{

    static Singleton instance=null;

    static readonly object padlock = new object();

    Singleton()

    {

    }

    public static Singleton Instance

    {

        get

        {

            lock (padlock)

            {

                if (instance==null)

                {

                    instance = new Singleton();

                }

                return instance;

            }

        }

    }

}

This implementation is thread-safe. The thread takes out a lock on a shared object, and then checks whether or not the instance has been created before creating the instance. This takes care of the memory barrier issue (as locking makes sure that all reads occur logically after the lock acquire, and unlocking makes sure that all writes occur logically before the lock release) and ensures that only one thread will create an instance (as only

one thread can be in that part of the code at a time - by the time the second thread enters it,the first thread will have created the instance, so the expression will evaluate to false). Unfortunately, performance suffers as a lock is acquired every time the instance is requested.

Note that instead of locking on typeof(Singleton) as some versions of this implementation do, I lock on the value of a static variable which is private to the class. Locking on objects which other classes can access and lock on (such as the type) risks performance issues and even deadlocks. This is a general style preference of mine - wherever possible, only lock on objects specifically created for the purpose of locking, or which document that they are to be locked on for specific purposes (e.g. for waiting/pulsing a queue). Usually such objects should be private to the class they are used in. This helps to make writing thread-safe applications significantly easier.

Reference: http://www.csharphelp.com/archives4/archive670.html