TranslateTransform for drag and drop in Silverlight - silverlight

Question

We're trying to implement drag and drop in Silverlight (3). We want users to be able to drag elements from a treeview onto another part of a UI. The parent element is a Grid, and we've been trying to use a TranslateTransform along with the MouseLeftButtonDown, MouseMove (etc) events, as recommended by various online examples. For example:
http://www.85turns.com/2008/08/13/drag-and-drop-silverlight-example/
We're doing this in IronPython, but that should be more or less irrelevant. The drag start is correctly initiated, but the item we are dragging appears in the 'wrong' location (offset a few hundred pixels to the right and down from the cursor) and I can't for the life of me work out why.
Basic xaml:
<Grid x:Name="layout_root">
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition Height="120"/>
</Grid.RowDefinitions>
<Border x:Name="drag" Background="LightGray" Width="40" Height="15"
Visibility="Collapsed" Canvas.ZIndex="10">
<Border.RenderTransform>
<TranslateTransform x:Name="transform" X="0" Y="0" />
</Border.RenderTransform>
<TextBlock x:Name="dragText" TextAlignment="Center"
Foreground="Gray" Text="foo" />
</Border>
...
</Grid>
The startDrag method is triggered by the MouseLeftButtonDown event (on a TextBlock in a TreeViewItem.Header). onDrag is triggered by MouseMove. In the following code self.root is Application.Current.RootVisual (top level UI element from app.xaml):
def startDrag(self, sender, event):
self.root.drag.Visibility = Visibility.Visible
self.root.dragText.Text = sender.Text
position = event.GetPosition(self.root.drag.Parent)
self.root.drag.transform.X = position.X
self.root.drag.transform.Y = position.Y
self.root.CaptureMouse()
self._captured = True
def onDrag(self, sender, event):
if self._captured:
position = event.GetPosition(self.root.drag.Parent)
self.root.drag.transform.X = position.X
self.root.drag.transform.Y = position.Y
The dragged item follows the mouse move, but is offset considerably. Any idea what I am doing wrong and how to correct it?

Answer 1

So it turns out that I should have been setting the Margin instead of using TranslateTransform:
def startDrag(self, sender, event):
self.root.drag.Visibility = Visibility.Visible
self.root.dragText.Text = sender.Text
self.root.CaptureMouse()
self._captured = True
self.root.MouseLeftButtonUp += self.stopDrag
self.root.MouseLeave += self.stopDrag
self.onDrag(sender, event)
def onDrag(self, sender, event):
if self._captured:
position = event.GetPosition(self.root.layout_root)
self.root.drag.Margin = Thickness(position.X, position.Y, 0, 0)
self.root.drag.UpdateLayout()