How Can I Reliably Cancel an Awaitable Task in WinRT?

Addressing Unreliable Task Cancellation in WinRT

Using CancelNotification to stop WinRT tasks can be unreliable; the method might appear successful, yet the task continues running. This often leads to a completed task status despite the cancellation attempt.

A Robust Cancellation Approach

The solution lies in understanding .NET cancellation and the Task-Based Asynchronous Pattern (TAP). TAP recommends using CancellationToken within asynchronous methods. The crucial step is to pass the CancellationToken to every cancellable method and incorporate regular checks within those methods.

Improved Code Example:

This revised code demonstrates reliable task cancellation using await:

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

  try
  {
    // Await the task; an exception is thrown if cancelled.
    await task;
  }
  catch (OperationCanceledException)
  {
    // Handle cancellation gracefully.
    Console.WriteLine("Task cancelled successfully.");
  }
}

private int slowFunc(int a, int b, CancellationToken cancellationToken)
{
  string someString = string.Empty;
  for (int i = 0; i < 1000000; i++)
  {
    someString += i.ToString(); // Simulate long-running operation
    cancellationToken.ThrowIfCancellationRequested();
  }
  return a + b;
}</code>

This code utilizes CancellationToken as follows:

• Exception Handling: await task throws an OperationCanceledException if the task is cancelled. This exception is caught and handled.
• Regular Cancellation Checks: cancellationToken.ThrowIfCancellationRequested() inside slowFunc regularly checks for cancellation requests.
• Graceful Cancellation: If cancellation is detected, ThrowIfCancellationRequested throws an exception, propagating the cancellation signal up the call stack.

This approach ensures reliable cancellation of awaited tasks in WinRT, preventing background processes and providing a more robust solution.

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