ListBoxItem.Parent returns nothing, unable to get it thru VisualTreeHelper.GetParent either

Question

How do I extract the parent container of a ListBoxItem? In the following example I can go till the ListBoxItem, higher than that I get Nothing:

<ListBox Name="lbAddress">
  <ListBox.ItemTemplate>
    <DataTemplate>
      <Button Click="Button_Click"/>
    </DataTemplate>
  </ListBox.ItemTemplate>
</ListBox>

---

Private Sub Button_Click(sender As Button, e As RoutedEventArgs)
  Dim lbAddress = GetAncestor(Of ListBox) 'Result: Nothing
End Sub

---

Public Shared Function GetAncestor(Of T)(reference As DependencyObject) As T
  Dim parent = GetParent(reference)

  While parent IsNot Nothing AndAlso Not parent.GetType.Equals(GetType(T))
    parent = GetAncestor(Of T)(parent)
  End While

  If parent IsNot Nothing Then _
    Return If(parent.GetType Is GetType(T), parent, Nothing) 

  Return Nothing    
End Sub

Public Function GetParent(reference As DependencyObject) As DependencyObject
  Dim parent As DependencyObject = Nothing

 If TypeOf reference Is FrameworkElement Then
    parent = DirectCast(reference, FrameworkElement).Parent
  ElseIf TypeOf reference Is FrameworkContentElement Then
    parent = DirectCast(reference, FrameworkContentElement).Parent
  End If

  Return If(parent, VisualTreeHelper.GetParent(reference))
End Function

Update

This is what happens (note that the 'parent' variable remains null):

Answer 1

The culprit appears to be your GetParent function, which uses the Parent property instead of VisualTreeHelper.GetParent. The Parent property returns the logical parent, not the visual parent, and will therefore return null when trying to traverse out of a DataTemplate. (It's also not clear how GetVisualAncestor is implemented -- or is this a typo for GetAncestor?) Use VisualTreeHelper.GetParent consistently, and forget about the Parent property. For example, the following code works for me:

XAML:

<DataTemplate x:Key="SimpleItemTemplate">
  <Button Click="Button_Click">In DataTemplate</Button>
</DataTemplate>

Code behind:

private void Button_Click(object sender, RoutedEventArgs e)
{
  Button btn = (Button)sender;
  ListBox lb = FindAncestor<ListBox>(btn);
  Debug.WriteLine(lb);
}

public static T FindAncestor<T>(DependencyObject from)
  where T : class
{
  if (from == null)
  {
    return null;
  }

  T candidate = from as T;
  if (candidate != null)
  {
    return candidate;
  }

  return FindAncestor<T>(VisualTreeHelper.GetParent(from));
}

When I run this, the ListBox variable (lb) in the Click handler is set to the correct non-null value.

Answer 2

The Parent property returns the logical parent. You should use the visual parent, because in some cases the logical parent will be null. For instance, in your case the Parent property of the button returns null.

From MSDN :

Parent may be null in cases where an element was instantiated, but is not attached to any logical tree that eventually connects to the page level root element, or the application object.

Since the FrameworkElement.VisualParent property is protected, you can use the VisualTreeHelper.GetParent method :

<System.Runtime.CompilerServices.Extension> _
Public Shared Function FindAncestor(Of T As DependencyObject)(ByVal obj As DependencyObject) As T
    Return TryCast(obj.FindAncestor(GetType(T)), T)
End Function

<System.Runtime.CompilerServices.Extension> _
Public Shared Function FindAncestor(ByVal obj As DependencyObject, ByVal ancestorType As Type) As DependencyObject
    Dim tmp = VisualTreeHelper.GetParent(obj)
    While tmp IsNot Nothing AndAlso Not ancestorType.IsAssignableFrom(tmp.[GetType]())
        tmp = VisualTreeHelper.GetParent(tmp)
    End While
    Return tmp
End Function

Answer 3

It is totally obvious something is removing an item from lbAddress.ItemsSource during the button click. The question is, what? A closer look at the image you posted reveals the answer. Here's the bug in your code:

My.Context.DeleteObject(context)
My.Context.SaveChanges()

   ...

btn.GetVisualAncestor(...)

The first two lines update your data model. This immediately removes the ListBoxItem from the visual tree. When when you call GetVisualAncestor later to find the ListBox it fails because the ListBoxItem no longer has a parent of any kind.

I'm sure the solution is now obvious to you: Simply find the ListBox ancestor before you delete the data from the data model and you'll be good to go.