call_once for C#

One of the useful gems that made it into C++11 Standard Template Libraries (STD) is call_once, this nifty little method makes sure that specific code is called only once (duh) and it follows these 3 rules:

Exactly one execution of exactly one of the functions (passed as f to the invocations in the group) is performed. It is undefined which function will be selected for execution. The selected function runs in the same thread as
thecall_once invocation it was passed to.

No invocation in the group returns before the abovementioned execution of the selected function is completed successfully, that is, doesn't exit via an exception.

If the selected function exits via exception, it is propagated to the caller. Another function is then selected and executed.

I needed something similar – I had a method that should only be called once (initialize) and I wanted to implement something similar to the call_once I’ve been using for my C++ development.
My first object was to try and make it as preferment as possible and so I’ve looked for a solution that does not involve locks:

public static class Call
{
    public static void Once(OnceFlag flag,  Action action)
    {
        if (flag.CheckIfNotCalledAndSet)
        {
            action.Invoke();            
        }
    }
}

since I was trying to mimic the C++ code I wrote two objects Call (above) and OnceFlag which has all of the magic inside using Interlocked:

public class OnceFlag
{
    private const int NotCalled = 0;
    private const int Called = 1;
    private int _state = NotCalled;
    internal bool CheckIfCalledAndSet
    {
        get
        {
            var prev = Interlocked.Exchange(ref _state, Called);
            
            return prev == NotCalled;
        }
    }

    internal void Reset()
    {
        Interlocked.Exchange(ref _state, NotCalled);
    }
}

I’m using Interlocked as a thread-safe way to check & set the value making sure that only once it would return true – try it:

class Program
{
    static OnceFlag _flag = new OnceFlag();
    static void Main(string[] args)
    {
        var t1 = new Thread(() => DoOnce(1));
        var t2 = new Thread(() => DoOnce(2));
        var t3 = new Thread(() => DoOnce(3));
        var t4 = new Thread(() => DoOnce(4));

        t1.Start();
        t2.Start();
        t3.Start();
        t4.Start();

        t1.Join();
        t2.Join();
        t3.Join();
        t4.Join();
    }

    private static void DoOnce(int index)
    {
        Call.Once(_flag, () => Console.WriteLine("Callled (" + index + ")"));
    }
}

It’s very simple solution unfortunately not entirely correct – the method used will only be called once, but requirements 2 & 3 were not implemented. Luckily for me I didn’t need to make sure that exception enable another call to pass through nor did I need to block other calls until the first call finishes.

But I wanted to try and write a proper implementation, unfortunately not as preferment due to the use of locks:

public static void Once(OnceFlagSimple flag, Action action)
{
    lock (flag)
    {        
        if (flag.CheckIfNotCalled)
        {
            action.Invoke();
    
            flag.Set();            
        }
    }
}

It works, and since I’m already using lock I can split the check and Set methods and use a bool value inside the flag instead of Interlocked.

All other threads are blocked due to lock until first finish running – check!
In case of exception other method can execute the once block – check!
If exited properly the block would only execute once – check!

But not very good performance due to locking on ever time the method gets called.

I’m still looking for a better way to implement call_once – it’s a good exercise in threading and I might find a cool new ways to use the classes under Threading or Task namespaces.

please let me know if you have a better implementation – that’s what the comments are for…

Labels: .NET, async, C#, Multi-threading