A reusable pattern to convert event into task

.net, async-await, c#, events, generics

Solution

It is possible with a helper class and a fluent-like syntax:

public static class TaskExt
{
    public static EAPTask<TEventArgs, EventHandler<TEventArgs>> FromEvent<TEventArgs>()
    {
        var tcs = new TaskCompletionSource<TEventArgs>();
        var handler = new EventHandler<TEventArgs>((s, e) => tcs.TrySetResult(e));
        return new EAPTask<TEventArgs, EventHandler<TEventArgs>>(tcs, handler);
    }
}


public sealed class EAPTask<TEventArgs, TEventHandler>
    where TEventHandler : class
{
    private readonly TaskCompletionSource<TEventArgs> _completionSource;
    private readonly TEventHandler _eventHandler;

    public EAPTask(
        TaskCompletionSource<TEventArgs> completionSource,
        TEventHandler eventHandler)
    {
        _completionSource = completionSource;
        _eventHandler = eventHandler;
    }

    public EAPTask<TEventArgs, TOtherEventHandler> WithHandlerConversion<TOtherEventHandler>(
        Converter<TEventHandler, TOtherEventHandler> converter)
        where TOtherEventHandler : class
    {
        return new EAPTask<TEventArgs, TOtherEventHandler>(
            _completionSource, converter(_eventHandler));
    }

    public async Task<TEventArgs> Start(
        Action<TEventHandler> subscribe,
        Action action,
        Action<TEventHandler> unsubscribe,
        CancellationToken cancellationToken)
    {
        subscribe(_eventHandler);
        try
        {
            using(cancellationToken.Register(() => _completionSource.SetCanceled()))
            {
                action();
                return await _completionSource.Task;
            }
        }
        finally
        {
            unsubscribe(_eventHandler);
        }
    }
}

Now you have a `WithHandlerConversion` helper method, which can infer type parameter from converter argument, which means you need to write `WebBrowserDocumentCompletedEventHandler` only one time. Usage:

await TaskExt
    .FromEvent<WebBrowserDocumentCompletedEventArgs>()
    .WithHandlerConversion(handler => new WebBrowserDocumentCompletedEventHandler(handler))
    .Start(
        handler => this.webBrowser.DocumentCompleted += handler,
        () => this.webBrowser.Navigate(@"about:blank"),
        handler => this.webBrowser.DocumentCompleted -= handler,
        CancellationToken.None);

Problem

I'd like to have a generic reusable piece of code for wrapping EAP pattern as task, something similar to what `Task.Factory.FromAsync` does for `BeginXXX/EndXXX` APM pattern. E.g.: ``` private async void Form1_Load(object sender, EventArgs e) { await TaskExt.FromEvent<EventArgs>( handler => this.webBrowser.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(handler), () => this.webBrowser.Navigate("about:blank"), handler => this.webBrowser.DocumentCompleted -= new WebBrowserDocumentCompletedEventHandler(handler), CancellationToken.None); this.webBrowser.Document.InvokeScript("setTimeout", new[] { "document.body.style.backgroundColor = 'yellow'", "1" }); } ``` So far, it looks like this: ``` public static class TaskExt { public static async Task<TEventArgs> FromEvent<TEventArgs>( Action<EventHandler<TEventArgs>> registerEvent, Action action, Action<EventHandler<TEventArgs>> unregisterEvent, CancellationToken token) { var tcs = new TaskCompletionSource<TEventArgs>(); EventHandler<TEventArgs> handler = (sender, args) => tcs.TrySetResult(args); registerEvent(handler); try { using (token.Register(() => tcs.SetCanceled())) { action(); return await tcs.Task; } } finally { unregisterEvent(handler); } } } ``` Is it possible to come up with something similar, which nevertheless would not require me to type `WebBrowserDocumentCompletedEventHandler` twice (for `registerEvent`/`unregisterEvent`), without resorting to reflection?

Original source

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