How to Properly Cancel Asynchronous Tasks in WinRT?

The correct way to cancel WinRT asynchronous tasks

In Windows 8 WinRT, tasks provide a mechanism to perform asynchronous operations. However, handling task cancellations can be tricky.

Consider the following code:

<code class="language-csharp">private async void TryTask()
{
    CancellationTokenSource source = new CancellationTokenSource();
    source.Token.Register(CancelNotification);
    source.CancelAfter(TimeSpan.FromSeconds(1));
    var task = Task<int>.Factory.StartNew(() => slowFunc(1, 2), source.Token);

    await task;

    if (task.IsCompleted)
    {
        MessageDialog md = new MessageDialog(task.Result.ToString());
        await md.ShowAsync();
    }
    else
    {
        MessageDialog md = new MessageDialog("Uncompleted");
        await md.ShowAsync();
    }
}

private int slowFunc(int a, int b)
{
    string someString = string.Empty;
    for (int i = 0; i < 1000000; i++)
    {
        someString += i.ToString();
    }
    return a + b;
}</code>

Despite the CancelNotification method being called, the task continues to run in the background and completes. In order to completely stop the task on cancellation, please follow these guidelines:

• Pass CancellationToken: Pass CancellationToken to every method that supports cancellation.
• Check CancellationToken periodically: This method must be checked CancellationToken periodically.

The improved code is as follows:

<code class="language-csharp">private async Task TryTask()
{
    CancellationTokenSource source = new CancellationTokenSource();
    source.CancelAfter(TimeSpan.FromSeconds(1));
    Task<int> task = Task.Run(() => slowFunc(1, 2, source.Token), source.Token);

    // (取消的任务在等待时会引发异常)。
    await task;
}

private int slowFunc(int a, int b, CancellationToken cancellationToken)
{
    string someString = string.Empty;
    for (int i = 0; i < 1000000; i++)
    {
        cancellationToken.ThrowIfCancellationRequested(); // 检查取消请求
        someString += i.ToString();
    }
    return a + b;
}</code>

With these modifications, if a task is canceled during execution, the task will be completely canceled and an exception will be thrown while waiting.

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