public void AddSurface(DropSurface surface)
 {
     if (!mSurfaces.Contains(surface))
     {
         if (TopSurface == null)
         {
             TopSurface = surface;
         }
         surface.Active = this.IsVisible;
         mSurfaces.Add(surface);
     }
 }
 public void RemoveSurface(DropSurface surface)
 {
     mSurfaces.Remove(surface);
 }