在 async void 中取消任务

Question

你好，我有一个列表框每当用户选择一个项目时，就会向网络发送一个请求现在我想取消用户选择项目然后开始之前的操作新操作。

我使用了以下代码来完成此操作，我想知道这些代码是否有效。或者我应该尝试其他方法？

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)
    {
      ....
    }
  }
}

和我运行这样运作

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

Answer 1

从技术上讲，您可以这样说，但是看一下：您 开火后忘记 ，让 return Task 让调用者知道是否 Subf2mCore 已完成、失败或 已取消 :

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);

用法：别忘了Dispose CancellationTokenSource 实例：

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

编辑: 如果你想从外部取消:

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;
  }
}

在 async void 中取消任务

Cancel Task in async void

c#

task

cancellation

cancellationtokensource