Best way to create a “run-once” time delayed function in C#

Question

I am trying to create a function that takes in an Action and a Timeout, and executes the Action after the Timeout. The function is to be non-blocking. The function must be

Answer 1

There is nothing built-in to .Net 4 to do this nicely. Thread.Sleep or even AutoResetEvent.WaitOne(timeout) are not good - they will tie up thread pool resources, I have been burned trying this!

The lightest weight solution is to use a timer - particularly if you will have many tasks to throw at it.

First make a simple scheduled task class:

class ScheduledTask
{
    internal readonly Action Action;
    internal System.Timers.Timer Timer;
    internal EventHandler TaskComplete;

    public ScheduledTask(Action action, int timeoutMs)
    {
        Action = action;
        Timer = new System.Timers.Timer() { Interval = timeoutMs };
        Timer.Elapsed += TimerElapsed;            
    }

    private void TimerElapsed(object sender, System.Timers.ElapsedEventArgs e)
    {
        Timer.Stop();
        Timer.Elapsed -= TimerElapsed;
        Timer = null;

        Action();
        TaskComplete(this, EventArgs.Empty);
    }
}

Then, create a scheduler class - again, very simple:

class Scheduler
{        
    private readonly ConcurrentDictionary _scheduledTasks = new ConcurrentDictionary();

    public void Execute(Action action, int timeoutMs)
    {
        var task = new ScheduledTask(action, timeoutMs);
        task.TaskComplete += RemoveTask;
        _scheduledTasks.TryAdd(action, task);
        task.Timer.Start();
    }

    private void RemoveTask(object sender, EventArgs e)
    {
        var task = (ScheduledTask) sender;
        task.TaskComplete -= RemoveTask;
        ScheduledTask deleted;
        _scheduledTasks.TryRemove(task.Action, out deleted);
    }
}

It can be called as follows - and is very lightweight:

var scheduler = new Scheduler();

scheduler.Execute(() => MessageBox.Show("hi1"), 1000);
scheduler.Execute(() => MessageBox.Show("hi2"), 2000);
scheduler.Execute(() => MessageBox.Show("hi3"), 3000);
scheduler.Execute(() => MessageBox.Show("hi4"), 4000);