What is a singleton in C#?

Question

What is a Singleton and when should I use it?

Answer 1

A singleton is a class which only allows one instance of itself to be created - and gives simple, easy access to said instance. The singleton premise is a pattern across software development.

There is a C# implementation "Implementing the Singleton Pattern in C#" covering most of what you need to know - including some good advice regarding thread safety.

To be honest, It's very rare that you need to implement a singleton - in my opinion it should be one of those things you should be aware of, even if it's not used too often.

Answer 2

You asked for C#. Trivial example:

public class Singleton
{
    private Singleton()
    {
        // Prevent outside instantiation
    }

    private static readonly Singleton _singleton = new Singleton();

    public static Singleton GetSingleton()
    {
        return _singleton;
    }
}

Answer 3

What it is: A class for which there is just one, persistent instance across the lifetime of an application. See Singleton Pattern.

When you should use it: As little as possible. Only when you are absolutely certain that you need it. I'm reluctant to say "never", but there is usually a better alternative, such as Dependency Injection or simply a static class.

Answer 4

another way to implement singleton in c#, i personally prefer this way because you can access the instance of the singeton class as a property instead of a method.

public class Singleton
    {
        private static Singleton instance;

        private Singleton() { }

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

        //instance methods
    }

but well, as far as i know both ways are considered 'right' so it's just a thing of personal flavor.

Answer 5

using System;
using System.Collections.Generic;
class MainApp
{
    static void Main()
    {
        LoadBalancer oldbalancer = null;
        for (int i = 0; i < 15; i++)
        {
            LoadBalancer balancerNew = LoadBalancer.GetLoadBalancer();

            if (oldbalancer == balancerNew && oldbalancer != null)
            {
                Console.WriteLine("{0} SameInstance {1}", oldbalancer.Server, balancerNew.Server);
            }
            oldbalancer = balancerNew;
        }
        Console.ReadKey();
    }
}

class LoadBalancer
{
    private static LoadBalancer _instance;
    private List<string> _servers = new List<string>();
    private Random _random = new Random();

    private static object syncLock = new object();

    private LoadBalancer()
    {
        _servers.Add("ServerI");
        _servers.Add("ServerII");
        _servers.Add("ServerIII");
        _servers.Add("ServerIV");
        _servers.Add("ServerV");
    }

    public static LoadBalancer GetLoadBalancer()
    {
        if (_instance == null)
        {
            lock (syncLock)
            {
                if (_instance == null)
                {
                    _instance = new LoadBalancer();
                }
            }
        }

        return _instance;
    }

    public string Server
    {
        get
        {
            int r = _random.Next(_servers.Count);
            return _servers[r].ToString();
        }
    }
}

I took code from dofactory.com, nothing so fancy but I find this far good than examples with Foo and Bar additionally book from Judith Bishop on C# 3.0 Design Patterns has example about active application in mac dock.

If you look at code we are actually building new objects on for loop, so that creates new object but reuses instance as a result of which the oldbalancer and newbalancer has same instance, How? its due to static keyword used on function GetLoadBalancer(), despite of having different server value which is random list, static on GetLoadBalancer() belongs to the type itself rather than to a specific object.

Additionally there is double check locking here

if (_instance == null)
            {
                lock (syncLock)
                {
                    if (_instance == null)

since from MSDN

The lock keyword ensures that one thread does not enter a critical section of code while another thread is in the critical section. If another thread tries to enter a locked code, it will wait, block, until the object is released.

so every-time mutual-exclusion lock is issued, even if it don't need to which is unnecessary so we have null check.

Hopefully it helps in clearing more.

And please comment if I my understanding is directing wrong ways.

Answer 6

A Singleton (and this isn't tied to C#, it's an OO design pattern) is when you want to allow only ONE instance of a class to be created throughout your application. Useages would typically include global resources, although I will say from personal experience, they're very often the source of great pain.

Answer 7

Whilst the there can only ever be one instance of a singleton, it is not the same as a static class. A static class can only contain static methods and can never be instantiated, whereas the instance of a singleton may be used in the same way as any other object.

Answer 8

It's a design pattern and it's not specific to c#. More about it all over the internet and SO, like on this wikipedia article.

In software engineering, the singleton pattern is a design pattern that is used to restrict instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects (say, five). Some consider it an anti-pattern, judging that it is overused, introduces unnecessary limitations in situations where a sole instance of a class is not actually required, and introduces global state into an application.

You should use it if you want a class that can only be instanciated once.

Answer 9

I use it for lookup data. Load once from DB.

public sealed class APILookup
    {
        private static readonly APILookup _instance = new APILookup();
        private Dictionary<string, int> _lookup;

        private APILookup()
        {
            try
            {
                _lookup = Utility.GetLookup();
            }
            catch { }
        }

        static APILookup()
        {            
        }

        public static APILookup Instance
        {
            get
            {
                return _instance;
            }
        }
        public Dictionary<string, int> GetLookup()
        {
            return _lookup;
        }

    }

Answer 10

What is a singleton :
It is a class which only allows one instance of itself to be created, and usually gives simple access to that instance.

When should you use :
It depends on the situation.

Note : please do not use on db connection, for a detailed answer please refer to the answer of @Chad Grant

Here is a simple example of a Singleton:

public sealed class Singleton
{
    private static readonly Singleton instance = new Singleton();

    // Explicit static constructor to tell C# compiler
    // not to mark type as beforefieldinit
    static Singleton()
    {
    }

    private Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            return instance;
        }
    }
}

You can also use Lazy<T> to create your Singleton.

See here for a more detailed example using Lazy<T>

Answer 11

Here's what singleton is: http://en.wikipedia.org/wiki/Singleton_pattern

I don't know C#, but it's actually the same thing in all languages, only implementation differs.

You should generally avoid singleton when it's possible, but in some situations it's very convenient.

Sorry for my English ;)

Answer 12

I know it's very late to answer the question but with Auto-Property you can do something like that:

public static Singleton Instance { get; } = new Singleton();

Where Singleton is you class and can be via, in this case the readonly property Instance.

Answer 13

Thread Safe Singleton without using locks and no lazy instantiation.

This implementation has a static constructor, so it executes only once per Application Domain.

public sealed class Singleton
{

    static Singleton(){}

    private Singleton(){}

    public static Singleton Instance { get; } = new Singleton();

}

Answer 14

E.X You can use Singleton for global information that needs to be injected.

In my case, I was keeping the Logged user detail(username, permissions etc.) in Global Static Class. And when I tried to implement the Unit Test, there was no way I could inject dependency into Controller classes. Thus I have changed my Static Class to Singleton pattern.

public class SysManager
{
    private static readonly SysManager_instance = new SysManager();

    static SysManager() {}

    private SysManager(){}

    public static SysManager Instance
    {
        get {return _instance;}
    }
}

http://csharpindepth.com/Articles/General/Singleton.aspx#cctor

Answer 15

Fourth version from Implementing the Singleton Pattern in C#

One shortcut you can take with this implementation (and only this one) is to just make instance a public static readonly variable, and get rid of the property entirely. This makes the basic skeleton code absolutely tiny!

public sealed class Singleton
{
    public static readonly Singleton Instance = new Singleton();

    static Singleton()
    {
    }

    private Singleton()
    {
    }
}