在WinForms / WPF中等待后如何继续执行？

Question

I am trying to gracefully discard long running operations which are in the essence a sequence of downloading data and updating the UI.

For example user selects an item and its details start being loaded from the server. But then he changes his mind and selects another item. The previous operation is cancelled, and the process starts again. Yet there is a possibility that data is already received and continuation is posted to the message loop for execution. The question is whether it is possible with async/await .

There is a blog post by Stephen Toub where he offers this extension:

public static async Task<T> WithCancellation<T>( 
    this Task<T> task, CancellationToken cancellationToken) 
{ 
    var tcs = new TaskCompletionSource<bool>(); 
    using(cancellationToken.Register( 
                s => ((TaskCompletionSource<bool>)s).TrySetResult(true), tcs)) 
        if (task != await Task.WhenAny(task, tcs.Task)) 
            throw new OperationCanceledException(cancellationToken); 
    return await task; 
}

In return statement return await task task is already completed.
If I am awaiting a task in UI thread:

var result = await task.WithCancellation(token);
...continuation...

The return await task; is executed in UI thread.

But does continuation run synchronously with continuation of awaited async method or is posted separately to the message loop?

Synchronous execution allows to avoid the situation when cancellation (user selects another item) occurs later than continuation appears in message queue. If this is not the case, then I have to check additionally:

var result = await task.WithCancellation(token);
token.ThrowIfCancellationRequested();
...continuation...

Answer 1

继续将异步运行，直到任务内部第一次等待，然后将继续执行任务。