How to block until an event is fired in c#

Question

After asking this question, I am wondering if it is possible to wait for an event to be fired, and then get the event data and return part of it. Sort of like this:

private event MyEventHandler event;
public string ReadLine(){ return event.waitForValue().Message; }
...
event("My String");
...elsewhere...
var resp = ReadLine();

Please make sure whatever solution you provide returns the value directly rather than getting it from something else. I'm asking if the method above is available in some way. I know about Auto/ManuelResetEvent, but I don't know that they return the value directly like I did above.

Update: I declared an event using MyEventHandler (which contains a Message field). I have a method in another thread called ReadLine waiting for the event to fire. When the event fires the WaitForValue method (part of the event handling scene) returns the event args, which contains the message. The message is then returned by ReadLine to whatever had called it.

The accepted answer to that question I asked was what I did, but it just doesn't feel quite right. It almost feels like something could happen to the data between the ManuelResetEvent firing and the program retrieving the data and returning it.

Update: The main problem with the Auto/ManualResetEvent is that it is too vulnerable. A thread could wait for the event, and then not give enough time for anyone else to get it before changing it to something else. Is there a way to use locks or something else? Maybe using get and set statements.

Answer 1

A very easy kind of event you can wait for is the ManualResetEvent, and even better, the ManualResetEventSlim.

They have a WaitOne() method that does exactly that. You can wait forever, or set a timeout, or a "cancellation token" which is a way for you to decide to stop waiting for the event (if you want to cancel your work, or your app is asked to exit).

You fire them calling Set().

Here is the doc.

Answer 2

If you're happy to use the Microsoft Reactive Extensions, then this can work nicely:

public class Foo
{
    public delegate void MyEventHandler(object source, MessageEventArgs args);
    public event MyEventHandler _event;
    public string ReadLine()
    {
        return Observable
            .FromEventPattern<MyEventHandler, MessageEventArgs>(
                h => this._event += h,
                h => this._event -= h)
            .Select(ep => ep.EventArgs.Message)
            .First();
    }
    public void SendLine(string message)
    {
        _event(this, new MessageEventArgs() { Message = message });
    }
}

public class MessageEventArgs : EventArgs
{
    public string Message;
}

I can use it like this:

var foo = new Foo();

ThreadPoolScheduler.Instance
    .Schedule(
        TimeSpan.FromSeconds(5.0),
        () => foo.SendLine("Bar!"));

var resp = foo.ReadLine();

Console.WriteLine(resp);

I needed to call the SendLine message on a different thread to avoid locking, but this code shows that it works as expected.

Answer 3

You can use ManualResetEvent. Reset the event before you fire secondary thread and then use the WaitOne() method to block the current thread. You can then have secondary thread set the ManualResetEvent which would cause the main thread to continue. Something like this:

ManualResetEvent oSignalEvent = new ManualResetEvent(false);

void SecondThread(){
    //DoStuff
    oSignalEvent.Set();
}

void Main(){
    //DoStuff
    //Call second thread
    System.Threading.Thread oSecondThread = new System.Threading.Thread(SecondThread);
    oSecondThread.Start();

    oSignalEvent.WaitOne(); //This thread will block here until the reset event is sent.
    oSignalEvent.Reset();
    //Do more stuff
}

Answer 4

If the current method is async then you can use TaskCompletionSource. Create a field that the event handler and the current method can access.

    TaskCompletionSource<bool> tcs = null;

    private async void Button_Click(object sender, RoutedEventArgs e)
    {
        tcs = new TaskCompletionSource<bool>();
        await tcs.Task;
        WelcomeTitle.Text = "Finished work";
    }

    private void Button_Click2(object sender, RoutedEventArgs e)
    {
        tcs?.TrySetResult(true);
    }

This example uses a form that has a textblock named WelcomeTitle and two buttons. When the first button is clicked it starts the click event but stops at the await line. When the second button is clicked the task is completed and the WelcomeTitle text is updated. If you want to timeout as well then change

await tcs.Task;

to

await Task.WhenAny(tcs.Task, Task.Delay(25000));
if (tcs.Task.IsCompleted)
    WelcomeTitle.Text = "Task Completed";
else
    WelcomeTitle.Text = "Task Timed Out";