public static void ClearSelection( 
   GraphicsLayer graphicsLayer
)

{
  //unselect the first element in the currently selected elements
  var elem = graphicsLayer.GetSelectedElements().FirstOrDefault();
  // Note: must be called on the QueuedTask
  {
    if (elem != null)
      //Unselect one element
      graphicsLayer.UnSelectElement(elem);

    //unselect all elements
    graphicsLayer.UnSelectElements();
    //equivalent to
    graphicsLayer.ClearSelection();
  }
}