static DockPanelSplitter()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(DockPanelSplitter),
new FrameworkPropertyMetadata(typeof(DockPanelSplitter)));
// override the Background property
BackgroundProperty.OverrideMetadata(typeof(DockPanelSplitter), new FrameworkPropertyMetadata(Brushes.Transparent));
// override the Dock property to get notifications when Dock is changed
DockPanel.DockProperty.OverrideMetadata(typeof(DockPanelSplitter),
new FrameworkPropertyMetadata(Dock.Left, new PropertyChangedCallback(DockChanged)));
}