Okay here’s a weird .NET Windows Forms error scenario. One of the applications had a scenario where an ObjectDisposedException was occurring, but for only one of the users. The other users that worked that the particular application functionality involved did not experience the error. I finally managed to reproduce the problem in the office by initiating the application task, clicking the “Create Job” button and then immediately going to another application and flipping between other windows that have nothing to do with the application. My guess is that the user that experiences the issue interacts with the system in a particular GUI manner that uncovers the problem.
The stack trace when the ObjectDisposedException looks like this:
System.Windows.Forms.dll!System.Windows.Forms.Control.CreateHandle() + 0x1ff bytes
System.Windows.Forms.dll!System.Windows.Forms.Control.Handle.get() + 0x77 bytes
System.Windows.Forms.dll!System.Windows.Forms.Control.PointToScreen(System.Drawing.Point p) + 0x38 bytes
System.Windows.Forms.dll!System.Windows.Forms.Button.OnMouseUp(System.Windows.Forms.MouseEventArgs mevent = {X = 36 Y = 7 Button = Left}) + 0x79 bytes
System.Windows.Forms.dll!System.Windows.Forms.Control.WmMouseUp(ref System.Windows.Forms.Message m, System.Windows.Forms.MouseButtons button, int clicks) + 0xf2 bytes
System.Windows.Forms.dll!System.Windows.Forms.Control.WndProc(ref System.Windows.Forms.Message m) + 0x544 bytes
System.Windows.Forms.dll!System.Windows.Forms.ButtonBase.WndProc(ref System.Windows.Forms.Message m) + 0x151 bytes
System.Windows.Forms.dll!System.Windows.Forms.Button.WndProc(ref System.Windows.Forms.Message m) + 0x2b bytes
System.Windows.Forms.dll!System.Windows.Forms.Control.ControlNativeWindow.OnMessage(ref System.Windows.Forms.Message m) + 0xd bytes
System.Windows.Forms.dll!System.Windows.Forms.Control.ControlNativeWindow.WndProc(ref System.Windows.Forms.Message m) + 0xd6 bytes The Button.MouseUp method attempts to identify where on the screen the mouse up event occurred when the button has already been disposed. Being able to reproduce the problem in the office, at least allowed me to identify which button was involved. The button click handler code is pretty standard for a button that does some work then closes the form:
private void HandleCreateClick( object sender, EventArgs e )
{
// Application specific processing goes here in the real code.
Close();
}After much experimentation using the debugger and Reflector, it looks like under normal circumstances the Close() causes a side effect of the button losing focus, which then clears internal System.Windows.Forms.Control flags which ensure that the OnMouseUp method doesn’t attempt to do any mouse-up event handling. For some reason, in my particular application scenario, the OnLostFocus method is not called and OnMouseUp processing attempts to get the mouse coordinates which then causes the ObjectDisposedException to be thrown. No matter what I tried, I could not reproduce the problem in a simplified project that could be forwarded onto Microsoft to look into. The following workaround was used to stop the problem happening in the application:
private void HandleCreateClick( object sender, EventArgs e )
{
// Application specific processing goes here in the real code.
// The following line is a workaround for a problem where the createButton's
// MouseUp event was throwing an ObjectDisposedException. In most scenarios
// where a button click event handler Closes the Form, a LoseFocus event
// resets the internal flags which causes the MouseUp event processing to
// skip any attempts to access button information. For some reason this
// createButton scenario would occassionally not get the LoseFocus event
// and cause an exception to be thrown. Setting the Enabled flag to false
// is a workaround which clears internal button flags so that no
// MouseUp processing occurs. This works around the problem.
createButton.Enabled = false;
Close();
}