Singleton implementation

Questions & Answers
1

Is this singleton implementation correct and thread-safe?

class Class
{
    public static readonly Class Instance;

    static Class()
    {
        Instance = new Class();
    }

    private Class() {}
}
2

You will probably need to make Class public, and you'd be better off using a property than a readonly variable (Makes it easier to swap how you're implementing the instance later if you need to), but otherwise it's fine

public class Class
{
    private static readonly Class _instance = new Class();
    public static Class Instance { get { return _instance; } }

    static Class(){}

    private Class() {}
}

Another way to do it is using the double check lock pattern, which is a lot faster with multithreaded systems if creation when the instance is accessed is a must:

public class Class
{
    private static volatile Class _instance;
    private static object _lock = new object();

    static Class() {} //Stops the lock being created ahead of time if it's not necessary

    public static Class Instance
    {
        get
        {
            if (_instance == null)
            {
                lock(_lock)
                {
                    if (_instance == null) _instance = new Class();
                }
            }
            return _instance;
        }
    }

    private Class() {}
}
3

If you need your singleton to be a MonoBehaviour, then you should look at the answer on this page:

http://answers.unity3d.com/questions/10696/singletons-with-coroutines

which details how you can correctly set up a MonoBehaviour singleton. But if you don't need it to be a MonoBehaviour, then this is the best thread-safe way of doing it:

EDIT: Posted correct code.

class Keyboard : Device
{
    static object _lock = new object();
    public Keyboard Instance
    {
        get
        {
            lock (_lock)
            {
                return _instance ?? _instance = new Keyboard();
            }
        }
    }
    private Keyboard _instance;

    private Keyboard() { }
}
4

Thread safe singletons in C#: http://www.yoda.arachsys.com/csharp/singleton.html

Discusses performance and double check locking.

5

I got a perfect answer on StackOverflow

http://stackoverflow.com/questions/3136008/singleton-implementation-correct

6

C#

public class Singleton
{

protected Singleton();

private sealed class SingletonCreator
{
    private static readonly Singleton instance = new Singleton();
    public static Singleton Instance { get { return instance; } }
}

public static Singleton Instance { get { return SingletonCreator.Instance; } }

}

7

This is my favourite one so far. To make it thread safe, just use lock as already stated elsewhere:

using UnityEngine;
 
/// <summary>
/// Be aware this will not prevent a non singleton constructor
///   such as `T myT = new T();`
/// To prevent that, add `protected T () {}` to your singleton class.
/// </summary>
public class Singleton<T> : MonoBehaviour where T : MonoBehaviour
{
	private static T _instance;
 
	private static object _lock = new object();
 
	public static T Instance
	{
		get
		{
			if (applicationIsQuitting) {
				Debug.LogWarning("[Singleton] Instance '"+ typeof(T) +
					"' already destroyed on application quit." +
					" Won't create again - returning null.");
				return null;
			}
 
			lock(_lock)
			{
				if (_instance == null)
				{
					_instance = (T) FindObjectOfType(typeof(T));
 
					if ( FindObjectsOfType(typeof(T)).Length > 1 )
					{
						Debug.LogError("[Singleton] Something went really wrong " +
							" - there should never be more than 1 singleton!" +
							" Reopenning the scene might fix it.");
						return _instance;
					}
 
					if (_instance == null)
					{
						GameObject singleton = new GameObject();
						_instance = singleton.AddComponent<T>();
						singleton.name = "(singleton) "+ typeof(T).ToString();
 
						DontDestroyOnLoad(singleton);
 
						Debug.Log("[Singleton] An instance of " + typeof(T) + 
							" is needed in the scene, so '" + singleton +
							"' was created with DontDestroyOnLoad.");
					} else {
						Debug.Log("[Singleton] Using instance already created: " +
							_instance.gameObject.name);
					}
				}
 
				return _instance;
			}
		}
	}
 
	private static bool applicationIsQuitting = false;
	/// <summary>
	/// When Unity quits, it destroys objects in a random order.
	/// In principle, a Singleton is only destroyed when application quits.
	/// If any script calls Instance after it have been destroyed, 
	///   it will create a buggy ghost object that will stay on the Editor scene
	///   even after stopping playing the Application. Really bad!
	/// So, this was made to be sure we're not creating that buggy ghost object.
	/// </summary>
	public void OnDestroy () {
		applicationIsQuitting = true;
	}
}