Web API 中的 CancellationToken 无法正常工作

Question

我有从用户那里获取 CancellationToken 的 Web Api，其中的方法（DoWork）也获取 CancellationToken：

[HttpPost]
public async Task<long> GetInfo(CancellationToken cancellationToken)
{
   long result = 0;
   bool notDone = true;
   Task<long> t = Task.Run(async () =>
   {

      if (cancellationToken.IsCancellationRequested)
         cancellationToken.ThrowIfCancellationRequested();

      while (notDone && !cancellationToken.IsCancellationRequested)
      {
         result = await DoWork(cancellationToken);
         notDone = false;
      }

      return result;
   }, cancellationToken);
   try
   {
      return await t;
   }
   catch (AggregateException e)
   {
      Debug.WriteLine("Exception messages:");
      foreach (var ie in e.InnerExceptions)
         Debug.WriteLine("   {0}: {1}", ie.GetType().Name, ie.Message);

      Debug.WriteLine("\nTask status: {0}", t.Status);
      throw;
   }
   catch (Exception ex)
   {
      throw;
   }
}

private Task<long> DoWork(CancellationToken token)
{
   long result = 0;
   bool notDone = true;

   Task<long> task = Task.Run(() =>
   {

      if (token.IsCancellationRequested)
         token.ThrowIfCancellationRequested();

      while (notDone && !token.IsCancellationRequested)
      {
         Thread.Sleep(8000);
         result = 2;
         notDone = false;
      }

      return result;
   }, token);

   return task;
}

我希望当用户取消请求时，它会中止 DoWork 方法并且不继续该功能，但是在发送异常之后，当“Thread.Sleep”完成时，DoWork 方法会继续。 用户可以像这种方法“cc”一样调用 API 服务，您可以看到它在 5 秒后取消，而在 DoWork 方法中“Thread.Sleep”是 9 秒。用户收到异常，但方法仍在运行。

private async Task<bool> cc()
{
   UriBuilder builder = new UriBuilder("http://localhost:12458/api/Test/GetInfo");

   ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
   HttpClient client = new HttpClient();
   System.Threading.CancellationTokenSource s = new System.Threading.CancellationTokenSource();
   s.CancelAfter(5000);
   try
   {
      var result = client.PostAsJsonAsync<model1>(builder.ToString(), new model1 { }, s.Token).Result;
      string tmp = result.Content.ReadAsStringAsync().Result;
      long ApiResult = JsonConvert.DeserializeObject<long>(tmp);
   }
   catch (TaskCanceledException ex)
   {
   }
   catch (OperationCanceledException ex)
   {
   }
   catch (Exception ex)
   {
   }
   finally
   {
      s.Dispose();
   }
   return false;
}

Answer 1

好的，在您的代码中，您也应该处理 CancellationToken.IsCancellationRequested。这不是什么魔法，你应该做这项工作。

    public void DoWork(CancellationToken ctsToken)
    {
        ctsToken.ThrowIfCancellationRequested();
        DoSomething();

        ctsToken.ThrowIfCancellationRequested();
        DoSomethingElse();

        // end so on with checking CancellationToken before every part of work
    }

你的任务应该是这样的

    Task<long> t = Task.Run(async () =>
    {
        cancellationToken.ThrowIfCancellationRequested();

        result = await DoWork(cancellationToken);
        notDone = false;

        cancellationToken.ThrowIfCancellationRequested();

        return result;
    }, cancellationToken);

Answer 2

当使用Thread.Sleep(8000) 时实际持有主线程8 秒，它无法检查取消令牌。您应该像这样使用带有取消令牌的 Task.Delay：

while (notDone && !token.IsCancellationRequested)
  {
     await Task.Delay(8000, token);
     result = 2;
     notDone = false;
  }

Task.Delay 检查取消令牌本身。