How Can I Properly Cancel Asynchronous Tasks in Windows 8 WinRT?

Gracefully Handling Asynchronous Task Cancellation in WinRT

Windows 8 WinRT's asynchronous task management, while powerful, presents challenges when canceling tasks. A common problem is the CancelNotification method firing, yet the task continues execution, leaving the task status incorrectly marked as "completed" instead of "canceled."

The Solution: Leveraging CancellationToken Effectively

The key to resolving this lies in correctly implementing the CancellationToken and adhering to the Task-Based Asynchronous Pattern (TAP) guidelines. This involves passing a CancellationToken to every cancellable method and regularly checking its status within those methods.

Revised Code Example:

This improved code snippet demonstrates the proper use of CancellationToken:

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);

    try
    {
        // A canceled task will throw an exception when awaited.
        await task;
    }
    catch (OperationCanceledException)
    {
        // Handle cancellation gracefully.
    }
}

private int slowFunc(int a, int b, CancellationToken cancellationToken)
{
    for (int i = 0; i < 1000000; i++)
    {
        cancellationToken.ThrowIfCancellationRequested(); // Check for cancellation
        // ... your long-running operation ...
    }
    return a + b;
}

This revised slowFunc method incorporates a cancellationToken.ThrowIfCancellationRequested() check within its loop. If cancellation is requested, an OperationCanceledException is thrown, effectively stopping the task and correctly setting its status to "canceled." The TryTask method now includes a try-catch block to handle this exception.

Outcome:

This approach ensures that tasks are completely halted upon cancellation, providing accurate task status reporting and preventing unintended background execution. Proper exception handling further enhances robustness.

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