Is the geometry of your graphic a point (MapPoint)?  Make sure you are returning map coords.  Also, not sure if this could be a problem, but you aren't specifying the ContentTemplate of your infoWindow...  
You need:  MyInfoWindow.ContentTemplate = TryCast(LayoutRoot.Resources("InfoWindowTemplate"),system.windows.datatemplate)
(sorry, I am a VB.NET guy :p)