The exception is this:
System.InvalidOperationException: Invoke or BeginInvoke cannot be called on a control until the window handle has been created.
First I'll explain the relations in my app. There's a form named MainForm and another form named AssetsForm. The MainForm is creating an instance of the AssetsForm in the MainForm's constrcutor but doesn't AssetsForm.Show() it yet.
There is class named AssetsSource which implements the IObservable and sends data for display to the AssetsForm which implements the IObserver. When AssetsForm receives data to display, it creates a BackgroundWorker which handles the data and update a TreeView.
I've implemented the following wrong code to handle the UI updates from the BackgroundWorker:
private void Invoke(Control control, Action action)
{
if (control.InvokeRequired)
{
control.BeginInvoke(action);
}
else
{
control.Invoke(action);
}
}
It's wrong because instead of Invoke(action) I should have written action(); But I will refer to this later. Anyway, an InvalidOperationException was thrown from the Invoke(action) line of code. I can infer that the InvokeRequired evaluated to FALSE, although I update the TreeView from a BackgroundWorker !!
In MSDN it is written about Control.Invoke:
The Invoke method searches up the control's parent chain until it finds a control or form that has a window handle if the current control's underlying window handle does not exist yet. If no appropriate handle can be found, the Invoke method will throw an exception.
What is the parent chain and what is the window handle ? When the window handle is created ? I guess all this had to do with the fact that the AssetsForm is closed.
When I removed that line and uses only action(); as it should be, the program doesn't crash.
When the AssetsForm is opened before the AssetsSource sends updates to AssetsForm, by debugging I can see that InvokeRequired is evaluated to TRUE and BeginInvoke of the TreeView updates itself.
To sum everything up, I don't understand why when the AssetsForm is closed, then the InvokeRequired is false and the UI update (TreeView) is allowed to be from the thread that didn't create the TreeView.
