Overcoming the 64 Kilobyte Limit for List Box Data

The information in this article applies to:

• Microsoft Windows Software Development Kit for Windows version 3.0

SUMMARY

This article discusses a technique to overcome the 64K size limit for strings in a list box. It does not discuss many of the implementation details of owner-draw list boxes. For more information about these implementation details, please query on the following words:

   prod(winsdk) and odlist

MORE INFORMATION

In a standard list box control, the data strings are stored in a global memory block that is allocated and managed by Windows. As the number of strings in the list box grows, Windows increases the size of the memory block as necessary to accommodate the data. However, this block is limited to 64K.

One method to overcome this barrier is to implement an owner-draw list box. For each string visible in the list box, Windows sends a WM_DRAWITEM message to the window procedure of the parent window of an owner-draw list box. When an application processes a WM_DRAWITEM message, it displays the string.

For discussion purposes, assume that the application has an index into a database of information. Each index identifies a string to be displayed in the list box. When the application creates the list box, it is important that the LBS_HASSTRINGS style NOT be specified. Although the list box displays strings, the data stored in the list box is a numeric index to the strings, not the strings themselves. Add the data to the list box using a loop with the following statements:

   SendMessage(hListbox, LB_ADDSTRING, 0, lMyLongIndex);

When the parent window's window procedure receives a WM_DRAWITEM message, run the following code:

   lMyLongIndex = (LONG)(((LPDRAWITEMSTRUCT)lParam)->itemData);
   GetString(lMyLongIndex, szString);

   ...

   // Other WM_DRAWITEM processing

   ...

   DrawText(hDC, szString, ...);

This method adds one level of indirection to each access of the list box data. By storing an index to each string in the list box, rather than storing the strings themselves, the amount of memory required to maintain the strings in the list box is dramatically reduced. A list box is limited to 64K of data. Storing a long value for each entry allows up to 8160 entries in the list box. Each entry requires 8 bytes (4 bytes for the long value and 4 bytes of overhead) and the list box itself requires 256 bytes for overhead.