c# - 嵌套的 BackgroundWorkers : RunWorkerCompleted calls on wrong thread?

Question

我正在处理需要调用更多异步任务的异步操作。我试图通过使用 BackgroundWorkers 来保持简单，结果是一个 BackgroundWorker 的 DoWork() 回调调用一个创建第二个 BackgroundWorker 的方法，就像这样(为了简洁起见减去错误检查和所有爵士乐):

class Class1
{
    private BackgroundWorker _worker = null;

    public void DoSomethingAsync()
    {
        _worker = new BackgroundWorker();
        _worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(_worker_RunWorkerCompleted);
        _worker.DoWork += new DoWorkEventHandler(_worker_DoWork);
        _worker.RunWorkerAsync();
    }

    void _worker_DoWork(object sender, DoWorkEventArgs e)
    {
        Class2 foo = new Class2();
        foo.DoSomethingElseAsync();
        while(foo.IsBusy) Thread.Sleep(0);  // try to wait for foo to finish.
    }

    void _worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        // do stuff
    }
}

class Class2
{
    private BackgroundWorker _worker = null;
    Thread _originalThread = null;

    public AsyncCompletedEventHandler DoSomethingCompleted;

    public bool IsBusy { get { return _worker != null && _worker.IsBusy; } }

    public void DoSomethingElseAsync()
    {
        _originalThread = Thread.CurrentThread;

        _worker = new BackgroundWorker();
        _worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(_worker_RunWorkerCompleted);
        _worker.DoWork += new DoWorkEventHandler(_worker_DoWork);
        _worker.RunWorkerAsync();
    }

    void _worker_DoWork(object sender, DoWorkEventArgs e)
    {
        // do stuff
    }

    void _worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        Debug.Assert(Thread.CurrentThread == _originalThread);  // fails

        // Assuming the above Assert() were excluded, the following event would be raised on the wrong thread.
        if (DoSomethingCompleted != null) DoSomethingCompleted(this, new AsyncCompletedEventArgs(e.Error, e.Cancelled, null));
    }
}

所以问题是，我希望 Class2._Worker_RunWorkerCompleted() 在调用 Class2.DoSomethingElseAsync() 的同一线程上执行。这永远不会发生 - 相反，回调在一个全新的线程上运行。

这是我的怀疑:Class1 的 _worker_DoWork() 永远不会返回，这意味着该线程永远不会返回到事件监听器，即使存在(我怀疑一个不存在)。另一方面，如果 _worker_DoWork() 确实返回，Class1 的 BackgroundWorker 将自动提前完成 - 它需要等待 Class2 完成工作才能完成其工作。

这引出了两个问题:

1. 我的怀疑是否正确？
2. 像这样嵌套异步操作的最佳方式是什么？我可以挽救 BackgroundWorker 方法，还是有其他更合适的技术？

Answer 1

如果 BackgroundWorker 在 UI 线程上创建，DoWork 将在线程池线程上运行，而 RunWorkerCompleted 将在 UI 线程上运行.

如果 BackgroundWorker 是在后台线程(即不是 UI 线程)上创建的，DoWork 仍将在线程池线程和 RunWorkerCompleted 还将在线程池线程上运行。

在你的情况下，由于你不能编码对任意(线程池)线程的调用，你将无法保证你想要的行为，尽管你可能想看看 System .Threading.SynchronizationContext.