非通用 TaskCompletionSource 或替代方案

Question

I'm working with an alert window (Telerik WPF) that is normally displayed asynchronously ( code continues running while it is open) and I want to make it synchronous by using async/await.

I have this working with TaskCompletionSource but that class is generic and returns an object like Task<bool> when all I want is a plain Task with no return value.

public Task<bool> ShowAlert(object message, string windowTitle)
{
    var dialogParameters = new DialogParameters { Content = message };

    var tcs = new TaskCompletionSource<bool>();
    dialogParameters.Closed += (s, e) => tcs.TrySetResult(true);

    RadWindow.Alert(dialogParameters);

    return tcs.Task;
}

The code that calls that method is

await MessageBoxService.ShowAlert("The alert text.")

How can I return a non-generic Task that functions similarly which I can await until the dialogParameters.Closed event fires? I understand that I could just ignore the bool that is being returned in this code. I am looking for a different solution than that.

Answer 1

The method can be changed to:

public Task ShowAlert(object message, string windowTitle)

Task<bool> inherits from Task so you can return Task<bool> while only exposing Task to the caller

Edit:

I found a Microsoft document, http://www.microsoft.com/en-us/download/details.aspx?id=19957 , by Stephen Toub titled 'The Task-based Asynchronous pattern' and it has the following excerpt that recommends this same pattern.

There is no non-generic counterpart to TaskCompletionSource<TResult>. However, Task<TResult> derives from Task, and thus the generic TaskCompletionSource<TResult> can be used for I/O-bound methods that simply return a Task by utilizing a source with a dummy TResult (Boolean is a good default choice, and if a developer is concerned about a consumer of the Task downcasting it to a Task<TResult>, a private TResult type may be used)

Answer 2

If you don't want to leak information, the common approach is to use TaskCompletionSource<object> and complete with a result of null . Then just return it as a Task .

Answer 3

.NET 5 has a non-generic TaskCompletionSource .

It was added in this pull request: https://github.com/dotnet/runtime/pull/37452/files#diff-4a72dcb26e2d643c337baef9f64312f3

Answer 4

Nito.AsyncEx实现了一个非通用的TaskCompletionSource类，归功于上面的 @StephenCleary 先生。

Answer 5

From @Kevin Kalitowski

I found a Microsoft document, http://www.microsoft.com/en-us/download/details.aspx?id=19957 , by Stephen Toub titled 'The Task-based Asynchronous pattern'

There is an example in this document which I think deals with the issue as Kevin points out. This is the example:

public static Task Delay(int millisecondsTimeout)
{
    var tcs = new TaskCompletionSource<bool>();
    new Timer(self =>
    {
        ((IDisposable)self).Dispose();
        tcs.TrySetResult(true);
    }).Change(millisecondsTimeout, -1);
    return tcs.Task;
}

At first I thought it was not good because you can't directly add the "async" modifier to the method without a compilation message. But, if you change the method slightly, the method will compile with async/await:

public async static Task Delay(int millisecondsTimeout)
{
    var tcs = new TaskCompletionSource<bool>();
    new Timer(self =>
    {
        ((IDisposable)self).Dispose();
        tcs.TrySetResult(true);
    }).Change(millisecondsTimeout, -1);
    await tcs.Task;
}

Edit: at first I thought I'd gotten over the hump. But, when I ran the equivalent code in my app, this code just makes the app hang when it gets to await tcs.Task;. So, I still believe that this is a serious design flaw in the async/await c# syntax.