PRB: OLE Drop Target Does Not Permit Drop

• The Microsoft Foundation Classes (MFC) included with:

      - Microsoft Visual C++ for Windows, versions 1.5, 1.51, 1.52, 1.52a
      - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2
  

SYMPTOMS

When you drag an OLE object over a given drop target window, the cursor feedback indicates that a drop is not allowed. However, the same object can be dropped on other drop targets successfully, and other objects can be dropped on the given drop target.

CAUSE

The object descriptor for the object indicates the size of the object is (0,0), and the drop target window's OnDragOver method is using the object rectangle to determine whether the object is within the client area of the window.

RESOLUTION

If the computed position rectangle for a data object is empty, inflate the rectangle to size (1,1) before testing whether the rectangle is within the client area of the drop target window.

STATUS

This behavior is by design.

MORE INFORMATION

Applications that register a window as an OLE drop target should probably check the position of a data object that is dragged over the window before permitting a drop to occur. For example, the MFC sample program Oclient performs the following check in its OnDragOver method:

   DROPEFFECT CMainView::OnDragOver(COleDataObject*,
      DWORD grfKeyState, CPoint point)
   {
      // adjust target rect by original cursor offset
      point -= m_dragOffset;
   
      // check for point outside logical area (in hatched region)
      // GetTotalSize() returns the size passed to SetScrollSizes
      CRect rectScroll(CPoint(0, 0), GetTotalSize());
   
      CRect rectItem(point,m_dragSize);
      rectItem.OffsetRect(GetDeviceScrollPosition());
   
      DROPEFFECT de = DROPEFFECT_NONE;
      CRect rectTemp;
      if (rectTemp.IntersectRect(rectScroll, rectItem))
      {
         //... figure out the drop effect
      }
   
      //...update drop effect
   
      return de;
   }

When the object position rectangle is inflated to size (1,1), IntersectRect returns TRUE and the correct drop effect is returned. In the previous example, you would inflate the rectangle by adding the following lines after the rectItem is declared:

   if (rectItem.IsRectEmpty())
   {
      // Some applications might have a null size in the object descriptor
      rectItem.InflateRect(1,1);
   }