在异步无效中取消任务

Question

Hi I have a listbox Whenever a user selects an item a request is sent to the web Now I want to cancel the previous operation when the user selected the item and then start the new operation.

I used the following codes to do this, I wanted to know if these codes work well. Or should I try another way?

private CancellationTokenSource ts = new CancellationTokenSource();
private async void Subf2mCore(CancellationToken ct)
{
  HtmlDocument doc = await web.LoadFromWebAsync(url);
   ...
  foreach (var node in doc)
  {
    if (!ct.IsCancellationRequested)
    {
      ....
    }
  }
}

and i run func this way

ts?.Cancel();
ts = new CancellationTokenSource();
Subf2mCore(ts.Token);

Answer 1

Technically, you can put it like this, but look: you fire and forget , let return Task for caller to know if Subf2mCore has been completed , failed or cancelled :

private async Task Subf2mCore(CancellationToken ct)
{
  HtmlDocument doc = await web.LoadFromWebAsync(url);
   ...
  foreach (var node in doc)
  {
    // Often we cancel by throwing exception: 
    // it's easy to detect that the task is cancelled by catching this exception
    // ct.ThrowIfCancellationRequested();

    // You prefer to cancel manually: 
    // your cancellation can be silent (no exceptions) but it'll be 
    // difficult for caller to detect if task completed or not 
    // (partially completed and cancelled)
    if (!ct.IsCancellationRequested)
    {
      ....
    }
  }
}

// If we don't want to cancel 
private async Task Subf2mCore() => Subf2mCore(CancellationToken.None);

Usage: do not forget to Dispose CancellationTokenSource instance:

using (CancellationTokenSource ts = new CancellationTokenSource()) {
  ...
  await Subf2mCore(ts.Token);
  ...
}

Edit: if you want to cancel from outside :

private CancellationTokenSource ts = null;

...

using (CancellationTokenSource _ts = new CancellationTokenSource()) {
  // previous task (if any) cancellation
  if (null != ts)
    ts.Cancel();

  // let cancel from outside
  ts = _ts;

  try {
    ...
    await Subf2mCore(_ts.Token);
    ...
  }
  finally {
    // task completed, we can't cancel it any more
    ts = null;
  }
}