Tracking the CurrentChanged event of an ICollectionView declared as a dependencyProperty - wpf

Question

I have an ICollectionView declared like so;
Public Shared ReadOnly DataIcvProperty As DependencyProperty = DependencyProperty.Register("DataIcv", GetType(ICollectionView), GetType(DataNavigator), New PropertyMetadata(Nothing))
<Description("The CollectionView (as an ICollectionView) to be passed to the DataNavigator control"), Category("Navigation Data Source")>
Public Property DataIcv As ICollectionView
Get
Return GetValue(DataIcvProperty)
End Get
Set(ByVal Value As ICollectionView)
SetValue(DataIcvProperty, Value)
End Set
End Property
On which I would like to keep track of the CurrentChanged event. So in the constructor of a wpf userControl (in which this property is declared I have added the following:
AddHandler DataIcv.CurrentChanged, AddressOf OnDataICVCurrentChanged
and then added the following
Public Sub OnDataICVCurrentChanged(ByVal sender As Object, e As EventArgs) Handles internalIcv.CurrentChanged
'Do whatever needs doing when the record in the ICollectionView changes
End Sub
When I then compile the usercontrol and use it in a separate project I get a NullReferenceException on the AddHandler line in the Constructor of the control (without any inner exception details).
What do I need to do to keep track of the current Changed event of the DataIcv so that I can have elements of my user control react properly to those changes?
Thanks
EDIT
This relates to the issue I described here. Essentially I want the navigator control to reflect the fact that the end user might select rows on the grid independently of the navigator.

Answer 1

The problem is, when you try to subscribe to the CurrentChanged event in your constructor, the DependencyProperty DataIcvProperty has a default value Nothing (as you specified in your property registration). So the NullReferenceException.
You could solve this with a PropertyChangeCallback (see documentation).
I come from the C# world and cannot guarantee the right syntax for VB.NET, but this approach surely will work:
Public Shared ReadOnly DataIcvProperty As DependencyProperty =
DependencyProperty.Register("DataIcv",
GetType(ICollectionView), GetType(DataNavigator),
New FrameworkPropertyMetadata(
Nothing,
New PropertyChangedCallback(AddressOf OnDataIcvChanged)))
Then you need to implement the OnDataIcvChanged static method, which will be called every time the property value changes.
Private Shared Sub OnDataIcvChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
If e.OldValue IsNot Nothing
RemoveHandler (e.OldValue As ICollectionView).CurrentChanged, AddressOf OnDataICVCurrentChanged
EndIf
If e.NewValue IsNot Nothing
AddHandler (e.NewValue As ICollectionView).CurrentChanged, AddressOf OnDataICVCurrentChanged
EndIf
End Sub
Update:
If your event handler method is not static, then you should access it via the object instance:
Private Shared Sub OnDataIcvChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
Dim DataNavigator instance = d As DataNavigator
If e.OldValue IsNot Nothing
RemoveHandler (e.OldValue As ICollectionView).CurrentChanged, AddressOf instance.OnDataICVCurrentChanged
EndIf
If e.NewValue IsNot Nothing
AddHandler (e.NewValue As ICollectionView).CurrentChanged, AddressOf instance.OnDataICVCurrentChanged
EndIf
End Sub
Edit: Code That eventually worked:
Public Shared ReadOnly DataIcvProperty As DependencyProperty = DependencyProperty.Register("DataIcv", GetType(ICollectionView), GetType(DataNavigator), New FrameworkPropertyMetadata(Nothing, New PropertyChangedCallback(AddressOf OnDataIcvChanged)))
<Description("The CollectionView (as an ICollectionView) to be passed to the DataNavigator control"), Category("Navigation Data Source")>
Public Property DataIcv As ICollectionView
Get
Return CType(GetValue(DataIcvProperty), ICollectionView)
End Get
Set(ByVal Value As ICollectionView)
SetValue(DataIcvProperty, Value)
End Set
End Property
Private Shared Sub OnDataIcvChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
Dim dn As DataNavigator = CType(d, DataNavigator)
If e.OldValue IsNot Nothing Then
RemoveHandler dn.DataIcv.CurrentChanged, AddressOf dn.OnDataICVCurrentChanged
End If
If e.NewValue IsNot Nothing Then
AddHandler dn.DataIcv.CurrentChanged, AddressOf dn.OnDataICVCurrentChanged
End If
End Sub
Private Sub OnDataICVCurrentChanged(ByVal sender As Object, ByVal e As EventArgs)
Record.Text = (DataIcv.CurrentPosition + 1).ToString
End Sub