Q139648: PRB: OLE Drop Target Does Not Permit Drop

Article: Q139648
Product(s): Microsoft C Compiler
Version(s): winnt:2.0,2.1,2.2
Operating System(s): 
Keyword(s): kbActiveX kbCOMt kbDragDrop kbMFC kbVC150 kbVC200 kbVC220 kbGrpDSMFCATL
Last Modified: 06-MAY-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.5, 1.51, 1.52, 1.52a 
   - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2 
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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;
     }

However, for some objects, such as Microsoft Word version 6.0 Documents, the
m_dragSize reported by the data object is (0,0). In this case, the call to
IntersectRect returns FALSE, and the drop effect returned is DROPEFFECT_NONE.

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);
     }

" (without the quotation marks)

Additional query words: 0Client drag/drop Word

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Keywords          : kbActiveX kbCOMt kbDragDrop kbMFC kbVC150 kbVC200 kbVC220 kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:2.0,2.1,2.2
Issue type        : kbprb

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