Avoid calling Invoke when the control is disposed

Question

I have the following code in my worker thread (ImageListView below is derived from Control):

if (mImageListView != null && 
    mImageListView.IsHandleCreated &&
    !mImageListView.IsDisposed)
{
    if (mImageListView.InvokeRequired)
        mImageListView.Invoke(
            new RefreshDelegateInternal(mImageListView.RefreshInternal));
    else
        mImageListView.RefreshInternal();
}

However, I get an ObjectDisposedException sometimes with the Invoke method above. It appears that the control can be disposed between the time I check IsDisposed and I call Invoke. How can I avoid that?

Answer 1

One way might be to call the method itself ones more instead of invoking the ImageListView-Method:

if (mImageListView != null && 
    mImageListView.IsHandleCreated &&
    !mImageListView.IsDisposed)
{
    if (mImageListView.InvokeRequired)
        mImageListView.Invoke(new YourDelegate(thisMethod));
    else
        mImageListView.RefreshInternal();
}

That way it would check one more time before finally calling RefreshInternal().

Answer 2

may be lock(mImageListView){...} ?

Answer 3

What you have here is a race condition. You're better off just catching the ObjectDisposed exception and be done with it. In fact, I think in this case it is the only working solution.

try
{
    if (mImageListView.InvokeRequired)
       mImageListView.Invoke(new YourDelegate(thisMethod));
    else
       mImageListView.RefreshInternal();
} 
catch (ObjectDisposedException ex)
{
    // Do something clever
}

Answer 4

There are implicit race conditions in your code. The control can be disposed between your IsDisposed test and the InvokeRequired test. There's another one between InvokeRequired and Invoke(). You can't fix this without ensuring the control outlives the life of the thread. Given that your thread is generating data for a list view, it ought to stop running before the list view disappears.

Do so by setting e.Cancel in the FormClosing event and signaling the thread to stop with a ManualResetEvent. When the thread completes, call Form.Close() again. Thread.Abort() is a distant second choice but much easier to implement. Using BackgroundWorker makes it easy to implement the thread completion logic.

Answer 5

You could use mutexes.

Somewhere at the start of the thread :

 Mutex m=new Mutex();

Then :

if (mImageListView != null && 
    mImageListView.IsHandleCreated &&
    !mImageListView.IsDisposed)
{
    m.WaitOne(); 

    if (mImageListView.InvokeRequired)
        mImageListView.Invoke(
            new RefreshDelegateInternal(mImageListView.RefreshInternal));
    else
        mImageListView.RefreshInternal();

    m.ReleaseMutex();
}

And whereever it is you are disposing of mImageListView :

 m.WaitOne(); 
 mImageListView.Dispose();
 m.ReleaseMutex();

This should ensure you cant dispose and invoke at the same time.

Answer 6

See also this question:

Avoiding the woes of Invoke/BeginInvoke in cross-thread WinForm event handling?

The utility class that resulted EventHandlerForControl can solve this problem for event method signatures. You could adapt this class or review the logic therein to solve the issue.

The real problem here is that nobugz is correct as he points out that the APIs given for cross-thread calls in winforms are inherently not thread safe. Even within the calls to InvokeRequired and Invoke/BeginInvoke themselves there are several race conditions that can cause unexpected behavior.