Silverlight中按钮实现简单的拖放功能注意点

<Canvas Background="#46461F">
<Button
MouseLeftButtonDown="OnMouseDown"
MouseMove="OnMouseMove"
MouseLeftButtonUp="OnMouseUp"
Canvas.Left="50" Canvas.Top="50" Background="Red"
FontSize="18"
ClickMode="Hover"
Width="160" Height="80">
<Button.Content>
<StackPanel Orientation="Horizontal" HorizontalAlignment="Center"
VerticalAlignment="Center">

<TextBlock Text="拖动我" VerticalAlignment="Center" Margin="10"></TextBlock>
</StackPanel>
</Button.Content>
</Button>
</Canvas>

红色为注意点，否则按钮触发不了MouseLeftButtonDown与MouseLeftButtonUp

以下为事件　

bool trackingMouseMove = false;
Point mousePosition;

void OnMouseDown(object sender, MouseButtonEventArgs e)
{
FrameworkElement element = sender as FrameworkElement;
mousePosition = e.GetPosition(null);
trackingMouseMove = true;
if (null != element)
{
element.CaptureMouse();
element.Cursor = Cursors.Hand;
}
}

void OnMouseMove(object sender, MouseEventArgs e)
{
FrameworkElement element = sender as FrameworkElement;
if (trackingMouseMove)
{
double deltaV = e.GetPosition(null).Y - mousePosition.Y;
double deltaH = e.GetPosition(null).X - mousePosition.X;
double newTop = deltaV + (double)element.GetValue(Canvas.TopProperty);
double newLeft = deltaH + (double)element.GetValue(Canvas.LeftProperty);

element.SetValue(Canvas.TopProperty, newTop);
element.SetValue(Canvas.LeftProperty, newLeft);

mousePosition = e.GetPosition(null);
}
}

void OnMouseUp(object sender, MouseButtonEventArgs e)
{
FrameworkElement element = sender as FrameworkElement;
trackingMouseMove = false;
element.ReleaseMouseCapture();

mousePosition.X = mousePosition.Y = 0;
element.Cursor = null;
}