Add Handler method not being called when Routed Event is attached

Question

I'm trying to create an attached behavior to add an auto-complete list to the standard TextBox control. My goal is that every time the TextChanged event is raised, my AutoCompleteBehavior class creates a popup and fills it with potential auto-complete results. To get these results, my AutoCompleteBehavior declares the following event:

Public Shared ReadOnly AutoCompleteListRequestedEvent As RoutedEvent =
                  EventManager.RegisterRoutedEvent("AutoCompleteListRequested",
                  RoutingStrategy.Bubble,
                  GetType(AutoCompleteListRequestedEventHandler), GetType(AutoCompleteBehavior))

The above is meant to be an attached event, used like this:

<TextBox lib:AutoCompleteBehavior.AutoCompleteListRequested="EventHandlerHere"/>

The idea is that when TextChanged is raised, AutoCompleteBehavior.AutoCompleteListRequested is also raised, which asks the implementing program to supply a list of suggestions for the current input.

For this to work, I have to hook in to the TextBox.TextChanged event as soon as my attached event is attached to said TextBox. Per Microsoft I should be able to declare a sub Add*Handler and Remove*Handler where the "*" is the name of the attached event, and these would be called whenever the attached event is added or removed from an element. So right below the event declaration I have:

        Public Shared Sub AddAutoCompleteListRequestedHandler(TB As TextBox, handler As AutoCompleteListRequestedEventHandler)
            'Code to hook into TextBox.TextChanged
        End Sub

        Public Shared Sub RemoveAutoCompleteListRequestedHandler(TB As TextBox, handler As AutoCompleteListRequestedEventHandler)
            'Code to unhook fromTextBox.TextChanged
        End Sub

My problem is AddAutoCompleteListRequestedHandler is never called. If I call TextBox.RaiseEvent to raise AutoCompleteListRequested, the event handler defined in XAML does get called (so the event is attached), but it seems my AddAutoCompleteListRequestedHandler is completely skipped.

As a final note, I found this question here which seems to be describing the same problem (my code is also in a dll just like his), but it's two years old and was never answered.

Answer 1

The XAML processor won't call your static methods when hooking up the event handler.

If you want to do something when the TextBox raises the TextChanged event, you would probably be better off implementing an attached behaviour and hook up to the TextChanged event in the PropertyChangedCallback or in the OnAttached() method depending on which kind of behaviour you create.

Please refer to my answer here for more information about attached behaviours.