Force redraw before long running operations

Question

When you have a button, and do something like:

Private Function Button_OnClick

    Button.Enabled = False

    [LONG OPERATION] 

End Function

Then the button will not be grayed, because the long operation prevents the UI thread from repainting the control. I know the right design is to start a background thread / dispatcher, but sometimes that's too much hassle for a simple operation.

So how do I force the button to redraw in disabled state? I tried .UpdateLayout() on the Button, but it didn't have any effects. I also tried System.Windows.Forms.DoEvents() which normally works when using WinForms, but it also had no effect.

Solution

The following code will do what you're looking for. However I would not use it. Use the BackgroundWorker class for long time operations. It's easy to use and very stable.
Here the code:

   public static void ProcessUITasks() {            
        DispatcherFrame frame = new DispatcherFrame();
        Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background, new DispatcherOperationCallback(delegate(object parameter) {
            frame.Continue = false;
            return null;
        }), null);
        Dispatcher.PushFrame(frame);
    }

Here you will find a sample on how to use the BackgroundWorker.

Comments

[HCL]

This will not work. Invalidation does only invalidate the region to be repainted. However it does not the painting.

[edvaldig]

@HCL, Works fine for me, see the documentation for the function: Invalidates the rendering of the element, and forces a complete new layout pass. OnRender is called after the layout cycle is completed.

[edvaldig]

He was pretty clear about being aware of but not using a Dispatcher/Backgroundworker :)

[HCL]

Are you shure your UI-thread is blocked? I have tested it the last time under .net3.5SP1. At this time it has not worked. If it works now, they must have changed the behaviour with 4.0. However, this would be a big surprise to me.

[HCL]

@edvaldig: I have underlined not to use it, however I have given him the code he was looking for. I don't think that this is worth a -1 from you. If my code does not work, then give me a -1, but not for giving some additional advice. I don't think this is fair.

[edvaldig]

You're right, I was too quick about it, comment upvote to compensate :)

[Eric Ouellet]

How do you use BackgroundWorker thread if you want to open another application through COM (which I think should be done on the UI thread / STA Apartment) ?

[Eric Ouellet]

Does reactive solve the problem of long operation on COM which should be done on the UI thread (STA apartment) ?