Question

How do I cancel all but the last/latest task? For example, suppose I have a time-consuming task that is triggered on a button click. I only want the task from the last button click to run and the previous ones to cancel. Can you show me how this is normally done?

My attempt involves storing all tasks in a list along with their cancellation token and removing them when either the task completes or is cancelled. Creating a list to store both Task and CancellationToken seems like I’m doing too much for what I would’ve thought is a common requirement (eg. a User does a search for something and clicks the search button multiple times. Shouldn’t only the last search be made and all other cancelled?). This is a common scenario, so I’d like to know how this is normally done. What is best practice here?

async void DoStuffAsync()
{
    // Store tasks in a list
    if (tasksAndCancelTokens == null)
        tasksAndCancelTokens = new List<Tuple<Task, CancellationTokenSource>>();
    else // we have a new Get request so cancel any previous
        tasksAndCancelTokens.ForEach(t => t.Item2.Cancel());

    // Create Cancellation Token
    var cts = new CancellationTokenSource();

    // Method to run asynchonously
    Func<int> taskAction = () =>
    {
        // Something time consuming
        Thread.Sleep(5000);
        if (cts.Token.IsCancellationRequested)
           cts.Token.ThrowIfCancellationRequested();
        return 100;
    };

    // Create Task
    Task<int> task = Task<int>.Factory.StartNew(taskAction, cts.Token);
    // Create Tuple to store task in list
    var tup = new Tuple<Task, CancellationTokenSource>(task, cts);
    tasksAndCancelTokens.Add(tup);

    try
    {
        int i = await task;
    }
    catch (OperationCanceledException)
    {
        // Don't need to do anything
    }
    finally
    {
        tasksAndCancelTokens.Remove(tup);
    }
}

Thanks

Answer 1

If you just want to cancel the last task, then that’s all you have to do:

CancellationTokenSource cts = null;
async void Button1_Click(...)
{
  // Cancel the last task, if any.
  if (cts != null)
    cts.Cancel();

  // Create Cancellation Token for this task.
  cts = new CancellationTokenSource();
  var token = cts.Token;

  // Method to run asynchonously
  Func<int> taskAction = () =>
  {
    // Something time consuming
    Thread.Sleep(5000);
    token.ThrowIfCancellationRequested();
    return 100;
  };

  try
  {
    int i = await Task.Run(taskAction);
  }
  catch (OperationCanceledException)
  {
    // Don't need to do anything
    return;
  }
}