停止BackgroundWorker启动的线程

Question

停止BackgroundWorker启动的线程

3

我有一个使用了BackgroundWorker的Windows表单。BackgroundWorker实例化了一个对象，然后执行该对象中的方法。我的问题是，当我使用backgroundworker.CancelAsync时，运行在远程对象上的方法并没有停止。在下面的示例中，dowork方法在单击取消按钮后继续执行。FYI，dowork正在遍历电子表格，并根据电子表格中的行进行一些数据操作。

    private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
    {
        myObject newObject = new myObject();
        newObject.dowork();

        if (backgroundWorker1.CancellationPending)
        {
            e.Cancel = true;
            backgroundWorker1.ReportProgress(0);
            return;
        }
    }

     private void btnCancel_Click(object sender, EventArgs e)
    {
       if (backgroundWorker1.IsBusy) backgroundWorker1.CancelAsync();
    }

你的想法是什么？

谢谢

- kurt

2

新对象.newObject.dowork()会检查backgroundWorker1.CancellationPending吗？而myObject newObject = new myObject();又是一个新线程吗？ - paparazzo

4

我确定newObject.dowork()是阻塞的，所以后台工作线程永远不会评估取消语句。你可以创建一个任务来代替使用后台工作线程，但取消同步方法可能很困难... - Ron Beyer

正如Blam所暗示的那样（读懂其中的含义），将BackgroundWorker的引用传递到你的myObject实例中（最好通过构造函数），并在循环内部检查CancellationPending标志。 - Idle_Mind

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- György Kőszeg · Accepted Answer

在btnCancel_Click中，您必须将取消请求传递给您的工作对象；否则，它将永远不会被通知。 BackgroundWorker.CancelAsync()仅简单地设置了BackgroundWorker.CancellationPending属性，通知BackgroundWorker的使用者（UI而非执行任务的人）您的任务已被取消。

因此，您需要：

MyObject myObject;

// This method is executed on the worker thread. Do not access your controls
// in the main thread from here directly.
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    myObject = new MyObject();

    // The following line is a blocking operation in this thread.
    // The user acts in the UI thread, not here, so you cannot do here
    // anything but wait.
    myObject.DoWork();

    // Now DoWork is finished. Next line is needed only to notify
    // the caller of the event whether a cancel has happened.
    if (backgroundWorker1.CancellationPending)
        e.Cancel = true;

    myObject = null;
}

private void btnCancel_Click(object sender, EventArgs e)
{
   if (backgroundWorker1.IsBusy)
   {
       backgroundWorker1.CancelAsync();

       // You must notify your worker object as well.
       // Note: Now you access the worker object from the main thread!
       // Note2: It would be possible to pass the worker to your object
       //        and poll the backgroundWorker1.CancellationPending from there,
       //        but that would be a nasty pattern. BL objects should not
       //        aware of the UI components.
       myObject.CancelWork();
   }
}

那么你应该如何实现通知：

public class MyObject
{
    // normally you should use locks to access fields from different threads
    // but if you just set a bool from one thread and read it from another,
    // then it is enough to use a volatile field.
    private volatile bool isCancelRequested;

    // this will be called from the main thread
    public void CancelWork()
    {
        isCancelRequested = true;
    }

    // This method is called from the worker thread.
    public void DoWork()
    {
        // Make sure you poll the isCancelRequested field often enough to
        // react to the cancellation as soon as possible.
        while (!isCancelRequested && ...)
        {
            // ...
        }
    }
}