PRB: Data Corruption with LBS_SORT and CCheckListBox on Win32s

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with:

      - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0,
        4.1 on the following platform: x86
  

SYMPTOMS

A CCheckListBox with the LBS_SORT style incorrectly copies the item data. Different strings in the listbox point to the same item data object. In debug mode using Visual C++ 4.0 and 4.1, this behavior will ASSERT in DBGHEAP.C as _free_dbg() is called on the same memory block multiple times.

This is only a problem in Win32s.

CAUSE

This is a problem in Win32s with owner-draw listboxes that have the LBS_HASSTRINGS and LBS_SORT listbox styles. CCheckListBox is an owner-draw listbox that uses LBS_OWNERDRAWFIXED or LBS_OWNERDRAWVARIABLE with LBS_HASSTRINGS. CCheckListBox uses the item data to maintain the check state information for each item.

RESOLUTION

To work around this behavior, avoid the LBS_SORT style or use InsertString() instead of AddString(). InsertString() does not cause a LBS_SORT list to be sorted.

If the listbox needs to be sorted, the strings should be sorted before being added to the listbox or the strings should be inserted in order.

STATUS

This behavior is by design.