Accessing UI Control from BackgroundWorker Thread

Question

I have a button on my windows form that calls the RunWorkerAsync() method, this in turn performs an action which then updates a ListBox on the same form.

After the DoWork event has finished I assign the Result for the event (Which is a list), I process the RunWorkerCompleted() event and then perform the following code to update my Listbox

which calls this:

(Apologies, code formatting won't work)

Now when I run the application and press the refresh button the following exception appears:

How would I get around this?

Edit:

The exception is thrown on the folowing statement, this occurs in the DoWork method where I clear the contents to keep the list up to date;

listBoxServers.Items.Clear();

Answer 1

Here's a snippet which I find very handy:

public static void ThreadSafe(Action action)
{
    Dispatcher.CurrentDispatcher.Invoke(DispatcherPriority.Normal, 
        new MethodInvoker(action));
}

You can pass it any delegate of Action type or simply a lambda like this:

ThreadSafe(() =>
{
    [your code here]
});

or

ThreadSafe(listBoxServers.Items.Clear);

Answer 2

You may not call Invoke on the list box, but on the form. For WinForms applications I use something like:

...
this.Invoke((MethodInvoker)delegate()
{
    // Do stuff on ANY control on the form.
});
...

Depending on the .NET version, you may have to declare a delegate for MethodInvoker yourself as

public delegate void MethodInvoker();

However, you might also consider using the ReportProgress feature of the Background Worker. The respective event handler should be called in the context of the form's thread.