LB_GETHORIZONTALEXTENT
wParam = 0; /* not used, must be zero */
lParam = 0L; /* not used, must be zero */An application sends the LB_GETHORIZONTALEXTENT message to retrieve from a list box the width, in pixels, by which the list box can be scrolled horizontally if the list box has a horizontal scroll bar.
This message has no parameters.
The return value is the scrollable width of the list box, in pixels.
To respond to the LB_GETHORIZONTALEXTENT message, the list box must have been defined with the WS_HSCROLL style.
This example gets the horizontal extent of a list box:
SendDlgItemMessage(hDlg, ID_MYLISTBOX, LB_GETHORIZONTALEXTENT, 0, 0L);