Changing a treeview item's image

Hello,

I'm trying to modify a tree view item's image dependant on whether it is expanded or not. The two images in the associated image list are side-by-side (Indexes 0 and 1), so it shouldn't be difficult to do it. Here's how I've tried to do it:-

Code:

(hwndTreeView = global tree view window handle)
(In my main window's WindowProc)
case WM_NOTIFY:
{
   switch ((LPNMHDR(lParam))->code)
   {
      case TVN_ITEMEXPANDED:
      {
         LPNMTREEVIEW pnmtv = (LPNMTREEVIEW)lParam;
         TVITEM tvi;
         
         tvi.hItem = pnmtv->itemNew.hItem;
         tvi.mask = TVIF_IMAGE;
         SendMessage(hwndTreeView, TVM_GETITEM, 0, (LPARAM)&tvi);
         if (pnmtv->itemNew.state & TVIS_EXPANDED)
            tvi.iImage++; //go from 0 to 1
         else
            tvi.iImage--;  //go from 1 to 0

         SendMessage(hwndTreeView, TVM_SETITEM, 0, (LPARAM)&tvi);

         break;
      }
   }

   break;
}

However, while the item's data does appear to be indeed modified through that process, the actual image does not change. I've tried setting the initial image to 1; The second image is displayed initially, and once the item is expanded it changes back to image 0, but it doesn't go the other way (If I collapse it again the image does not change). Something which changes does not seem to be acted upon properly, and it's really annoying.

I'll open the floor to Q&A now