Tuesday, December 15, 2009

WPF UserControl IsEnabled in WinForms host

While debugging today, I ran into something rather odd.  Two things actually.  And since I'm falling behind in my posting I thought I'd share.

Here's what I was dealing with.  I have a WinForms UserControl which contains a WPF UserControl within an ElementHost.  The WPF control is being informed of what is selected on the form and is Enabling or Disabling itself accordingly.  However, there is a special case in which the form knows it wants the control to be disabled all the time.

Before I go any further.  If you're working with WPF controls hosted in WinForms and you're dealing with enabling and disabling things, you should be aware of this bug and this workaround on Microsoft Support.  It will cause all kinds of havoc if you set things to Enabled=false before showing them.

The WPF user control is using MVVM, so it's IsEnabled property was bound to an IsEnabled property on the View-Model.  This is where we run into the first interesting thing: this doesn't work.  The vm.IsEnabled property was changed, and the PropertyChanged event was fired with "IsEnabled" as the property name, but the WPF user control's IsEnabled property did not change.  I found someone else who had this same issue and posted about it on this blog.  His work around cracks me up, I can't believe it works...  WPF is crazy.

My work around was to just hook the PropertyChanged event myself like this:
void _vm_PropertyChanged( object sender, PropertyChangedEventArgs e )
  if ( e.PropertyName == "IsEnabled" )
    this.IsEnabled = _vm.IsEnabled;
I was stunned when this code didn't work. It fired, it set the property, but after setting it the property value didn't change. And it didn't change after WPF got a chance to run through a layout pass either.

This is the second interesting thing.  Turns out the WinForm's control that the WPF control was in was disabled. Why?  Because the parent form had disabled it due to that "special case" I mentioned.  So when I set the WPF user control's IsEnabled to true it didn't matter because it's parent's Enabled property was false. So it just shrugged it's shoulders and ignored me.

So before I can enable the WPF control, I need to enable it's parent, the WinForms control.

In order to get this whole mess working what I ended up doing looks something like this:
  1. View-Model fires change event for IsEnabled
  2. WPF User Control fires custom change event for IsEnabled (it does NOT try to set IsEnabled)
  3. WinForms User Control sets Enabled = CustomEnabled from WPF User Control
  4. WinForms User Control's EnabledChanged fires
  5. WinForms User Control sets WPF UserControl's IsEnabled property = this.Enabled
  6. WPF User Control's IsEnabledChanged fires
  7. WPF User Control sets View-Model's IsEnabled = this.IsEnabled
This ensures that all Enabled properties on all the objects involved here will always be in sync, no matter which one you change.

Part of the problem here I'm going to blame on bad design.  I don't think the control should be being enabled and disabled in two completely different ways (one from the top (WinForms), and one from the bottom (View-Model)).  The rest of the problem I'm going to blame on Enabled properties being really confusing.  They have mystical relationships with their parents and their values can change at different times.  Most properties when you give them a value either have that value after the set, or throw an exception.  But not Enabled!  It's mystical.  I find that I know this but that I still get bit by it when I'm not expecting it.

1 comment:

  1. I'm SO glad you published this article! I was driving myself crazy to understand what was going on, and thanks to you I solved it.

    Great post!!! :)


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