When a selected IME needs a string for re-conversion, this message is sent to the application through the WM_IME_REQUEST message.
lParam = (LPARAM) lpRECONVERTSTRING;
Returns the required size of the RECONVERTSTRING if the application filled the RECONVERTSTRING structure in lParam. Otherwise, returns zero. However, if lParam is NULL, the application must return the required size of the RECONVERTSTRING.
Input Method Editor Overview, Input Method Editor Messages, RECONVERTSTRING, WM_IME_REQUEST