Common Pattern to Initialize A Singleton On A Background Thread

Question

I currently have a singleton, which can take up to 10 seconds to initialize. However, I don't want my users to penalized (waiting) for this initialize, so I'd rather bootstrap this component on a background thread during application startup. Here's what I have:

Singleton:

public class MySingleton
{
    private static MySingleton _instance;
    private static readonly object _locker = new object();

    private MySingleton() 
    {
        Init();
    }
    public static MySingleton Instance 
    { 
        if(_instance == null) _instance = new MySingleton();
        return _instance;
    }
    public void Init()
    {
        lock(_locker)
        {
            if(_instance != null) return;

            // long running code here...
        }
    }
}

Application Startup:

Task.Factory.StartNew(() => MySingleton.Instance.Init());

This code does work, guards against double init, guards against the edge case of the user needing it before it's done initializing and also guards against someone forgetting to call Init().

However, it feels a little clunky for two reasons: a) I'm going to go into the Init method twice on startup. b) I'd like to do threading inside the singleton, but something has to initiate the initialization.

Is there a cleaner/nicer/better way to handle this?

Thanks in advance for everyone's help.

**EDIT: As pointed out in the comments, Init was mistakenly scoped as private. It should be public and has been corrected.

Answer 1

Use the static constructor to trigger it and a ManualResetEvent for the syncing. It gives you a solution where everything is done within the actual class. It's therefore not dependent of that someone should call your init method.

public class MySingleton
{
    private static MySingleton _instance;
    private static ManualResetEvent _initEvent = new ManualResetEvent(false);

    static MySingleton() 
    {
        ThreadPool.QueueUserWorkItem(state => Init());
    }

    public static MySingleton Instance 
    {
        _initEvent.Wait();
        return _instance;
    }
    private static void Init()
    {
        _instance = new MySingleton();
        // long running code here...


        _initEvent.Set();
    }
}

The event will stay signaled once triggered which means that the Instance property will return ASAP when the Init method is done.