如何正確處理帶有MessageBox確認的PreviewMouseDown事件？

早些時候我問了how to cancel a WPF TreeViewItem.Selected event。如何正確處理帶有MessageBox確認的PreviewMouseDown事件？

的應答者建議我，而不是處理PreviewMouseDown事件的選擇，甚至發生之前。這就說得通了。

我一直試圖做...

XAML ...

<TreeView Name="TreeViewThings" 
    ... 
    PreviewMouseDown="TreeViewThings_PreviewMouseDown" 
    TreeViewItem.Expanded="TreeViewThings_Expanded" 
    TreeViewItem.Selected="TreeViewThings_Selected" >

的Visual Basic ...

 
Sub TreeViewThings_PreviewMouseDown(...) 
    If UnsavedChangesExist() Then 
     e.Handled = UserCancelled() 
    Else 
     e.Handled = False 
    End If 
End Sub 

Function UnsavedChangesExist() As Boolean 
    ... 
End Function 

Function UserCancelled() As Boolean 
    Return MessageBox.Show("Discard your unsaved changes?", _ 
          "Unsaved Changes", _ 
          MessageBoxButton.OKCancel, _ 
          MessageBoxImage.Question) = MessageBoxResult.Cancel 
End Function

這只是排序的工作...