An application sends a WM_GETTEXTLENGTH message to determine the length (in characters) of the text associated with a window. The length does not include the null-terminating character.
wParam
This parameter is not used.
lParam
This parameter is not used.
For an edit control, the text to be copied is the content of the edit control. For a combo box control, the text is the content of the edit control (or static text) portion of the combo box. For a button control, the text is the button name. For other windows, the text is the window caption. To determine the length of an item in a list box, an application can use the LB_GETTEXTLEN message.
LB_GETTEXTLEN, WM_GETTEXT