调用线程无法访问该对象，因为其他线程拥有它

Question

Ok, my current code is work in progress and probably I will try to do the same thing with the Async CTP. But I will still like to understand whats happening.

I have a function like below

// In MainWindow.xaml.cs
Task.Factory.StartNew(() => helper.Send());

// In class HttpHelper
public void Send()
{
    // ...

    try
    {
        Status = Statuses.Uploading;
        // write to request stream

        Status = Statuses.Downloading;
        // write to response stream

        Status = Statuses.Idle; // the exception is thrown here
        // ...
    }
    catch (Exception e)
    {
        // ...
    }
}

Full Code for HttpHelper @pastebin . Send() on line 76

I wonder why I get the exception? Maybe I did something wrong with the threading, but why is the exception raised only after I successfully set the Status property 2 times?

UPDATE: The Cause ...

I had an event handler, listening to the StatusChanged event, in 1 if clause, I forgot to use the UI thread to update the UI

helper.StatusChanged += (s, evt) =>
{
    _dispatcher.Invoke(new Action(() => txtStatus.Text = helper.Status.ToString())); // I used _dispatcher here correctly

    if (helper.Status == HttpHelper.Statuses.Idle || helper.Status == HttpHelper.Statuses.Error)
       progBar.IsIndeterminate = false; // but not here
};

Answer 1

Update : I missed the (in my defense, small) comment in your code that gave away that you're using WPF/Silverlight and not Windows Forms.

Still, looks like you were able to take the basic gist of what I said and apply it to your own scenario properly (using Dispatcher.Invoke rather than Control.Invoke )—well done ;)

---

I'm guessing an event handler attached to your StatusChanged event updates a StatusLabel control on your UI? If that's the case, then you need to Invoke that call from the UI thread; eg:

void HttpHelper_StatusChanged(object sender, EventArgs e)
{
    var httpHelper = (HttpHelper)sender;
    UpdateStatus(httpHelper.Status);
}

void UpdateStatus(HttpHelper.Statuses status)
{
    if (InvokeRequired)
    {
        Invoke(new Action<HttpHelper.Statuses>(UpdateStatus), status);
    }
    else
    {
        // Your code probably doesn't look like this;
        // it's just an example.
        statusLabel.Text = status.ToString();
    }
}

The reason you might see two successes followed by a failure is a bit beyond me; but I know that the StatusLabel control in particular can be a bit evasive when it comes to threading issues. I have seen code where it is updated directly from background threads (typically due to developer obliviousness) without any exception; it looks to me like you just got lucky twice and unlucky once.

Answer 2

Does Status touch the UI? Because it seems like it.

I can only guess but I imagine Status changes something on UI which needs to be done through Dispatcher.Invoke() .

Answer 3

if Status is a global variable you will get this exception becuase multiple threads are accessing and updating it at the same time. i suggest to use a lock around it