c# - TaskFactory.StartNew() 中的 "cancellationToken"有什么用？

Question

http://msdn.microsoft.com/en-us/library/dd988458.aspx

更新:

所以，让我们讨论这篇文章:http://msdn.microsoft.com/en-us/library/dd997396.aspx

我稍微修改了一下代码:

    static void Main()
    {

        var tokenSource2 = new CancellationTokenSource();
        CancellationToken ct = tokenSource2.Token;

        var task = Task.Factory.StartNew(() =>
        {

            // Were we already canceled?
            ct.ThrowIfCancellationRequested();

            bool moreToDo = true;
            Thread.Sleep(5000);
            while (moreToDo)
            {

                // Poll on this property if you have to do
                // other cleanup before throwing.
                if (ct.IsCancellationRequested)
                {
                    Console.WriteLine("exit");
                    // Clean up here, then...
                    ct.ThrowIfCancellationRequested();
                }

            }
        }, tokenSource2.Token); // this parameter useless

        Console.WriteLine("sleep");
        Thread.Sleep(2000);
        Console.WriteLine("cancel");

        tokenSource2.Cancel();

        // Just continue on this thread, or Wait/WaitAll with try-catch:
        try
        {
            task.Wait();
        }
        catch (AggregateException e)
        {
            foreach (var v in e.InnerExceptions)
            {
                Console.WriteLine(e.Message + " " + v.Message);
            }
        }

        Console.ReadKey();
    }

UPD:好吧，这只会改变 task.IsCanceled，恕我直言，这是无用的，因为我仍然应该手动实现所有内容。

Answer 1

由于评论，我发布了另一个答案。

考虑以下代码:

var tokenSource = new CancellationTokenSource();
CancellationToken ct = tokenSource.Token;

tokenSource.Cancel(); 

var task = Task.Factory.StartNew(() =>
{    
  // Were we already canceled?
  ct.ThrowIfCancellationRequested();
  // do some processing
});

即使调用 tokenSource.Cancel() 是在任务实际开始之前发出的，您仍然会从线程池中分配一个工作线程，因此您会浪费一些系统资源。

但是当您在 Task.Factory.StartNew 中指定 token 参数时，任务将立即取消，而不分配工作线程。