Hi nyinyithann,

you can attach to XamMap's ElementHover and ElementUnhover and control manually group of contries that should be highlighted. Try the following code snippet:

xamMap.ElementHoverAction = ElementHoverAction.None;
xamMap.ElementHover += xamMap_ElementHover;
xamMap.ElementUnhover += xamMap_ElementUnhover;

private List<MapElement> _highlightedElements = new List<MapElement>(3);

void xamMap_ElementUnhover(object sender, MapElementHoverEventArgs e)
{
 foreach (var element in _highlightedElements)
 {
  element.Fill = null;
 }

 _highlightedElements.Clear();
}

private void xamMap_ElementHover(object sender, MapElementHoverEventArgs e)
{                       
 if (e.Element.Caption == "United States")
 {
  _highlightedElements.Add(e.Element);
  _highlightedElements.Add(xamMap.Layers[0].Elements["Mexico"].Single());
  _highlightedElements.Add(xamMap.Layers[0].Elements["Canada"].Single());

  foreach (var element in _highlightedElements)
  {
   element.Fill = new SolidColorBrush(Colors.Orange);
  }
 }           
}

Please let me know if this works for you.

Regards,

Ivan Kotev