Q67678: List Box and Combo Box Capacity Limits

Article: Q67678
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1 
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SUMMARY
=======

In the Microsoft Windows graphical environment, the storage capacity of a list
box or combo box is limited by the total amount of text. Each item is stored in
memory as a NULL-terminated string.

The maximum amount of memory that a list box or combo box can use to store text
depends on the version of Windows in use. In Windows versions 2.x, the amount of
free memory in the user module's data segment limits available storage.

Beginning with Windows 3.0, each list box or combo box has its own data segment,
which allows it to store 64K of text. If a list box or combo box in an
application must store more text, the application can store up to 8160 items in
an owner-draw list box or combo box that does not have the LBS_HASSTRINGS list
box or CBS_HASSTRINGS combo box style. In this case, the application stores and
manages the item text separately.

MORE INFORMATION
================

An owner-draw list box or owner-draw combo box is limited to 64K of data
storage, as are regular (nonowner draw) list boxes and combo boxes. Because the
application manages the text when the list box or combo box do not have the
LBS_HASSTRINGS or CBS_HASSTRINGS styles, each item in an owner-draw list box or
owner-draw combo box require 8 bytes, 4 of which store the application-supplied
32-bit item-identifier value.

The list box (or combo box) uses 256 bytes of the 64K segment for internal
storage, and uses the rest for items. This yields room for 8160 items, as
follows:

  (64K segment - 256 bytes) / (8 bytes per item)

NOTE: In a list box with the LBS_HASSTRINGS style or combo box with the
CBS_HASSTRINGS style, the text pointed to by the pointer passed in the lParam of
LB_ADDSTRING or CB_ADDSTRING (or LB_INSERTSTRING or CB_INSERTSTRING) is stored
in the list box's or combo box's storage area. An additional 32-bit storage
value can be associated with each item with the LB_SETITEMDATA or CB_SETITEMDATA
styles.

When the LBS_HASSTRINGS or CBS_HASSTRINGS styles are not present, Windows no
longer stores text for the list box or combo box. Instead of storing the text
pointed to by the lParam of LB_ADDSTRING or CB_ADDSTRING (or LB_INSERTSTRING or
CB_INSERTSTRING), only the 32-bit value itself is stored, thus dramatically
reducing the needed storage. When the owner-draw list box or owner-draw combo
box receive the WM_DRAWITEM message, it is up to the drawing routine to
interpret the 32-bit value.

Additional query words: no32bit 3.00 3.10 listbox combobox list-box combo-box

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

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