The WM_MENUSELECT message is sent to a menu's owner window when the user selects a menu item. 

WM_MENUSELECT  
uItem = (UINT) LOWORD(wParam);   // menu item or submenu index 
fuFlags = (UINT) HIWORD(wParam); // menu flags 
hmenu = (HMENU) lParam;          // handle of menu clicked 
 

Parameters

uItem

Value of the low-order word of wParam. If the selected item is a command item, this parameter contains the identifier of the menu item. If the selected item opens a drop-down menu or submenu, this parameter contains the menu index of the drop-down menu or submenu in the main menu, and the hMenu parameter then contains the handle of the main (clicked) menu; use the GetSubMenu function to get the menu handle of the drop-down menu or submenu. 

fuFlags

Value of the high-order word of wParam. Specifies one or more menu flags. This parameter can be a combination of the following values: 

Value	Description
MF_BITMAP	Item displays a bitmap.
MF_CHECKED	Item is checked.
MF_DISABLED	Item is disabled.
MF_GRAYED	Item is grayed.
MF_HILITE	Item is highlighted.
MF_MOUSESELECT	Item is selected with the mouse.
MF_OWNERDRAW	Item is an owner-drawn item.
MF_POPUP	Item opens a drop-down menu or submenu.
MF_SYSMENU	Item is contained in the window menu (also known as the System menu or Control menu). The hmenu parameter identifies the window menu associated with the message.
 

hmenu

Value of lParam. Identifies the menu that was clicked. 

Return Values

If an application processes this message, it should return zero. 

Remarks

If the fuFlags parameter contains 0xFFFF and the hmenu parameter contains NULL, Windows has closed the menu. 
Do not use the value - 1  for fuFlags. That is because fuFlags is specified as (UINT) HIWORD(wParam). If HIWORD(wParam) were 0xFFFF, fuFlags (because of the UINT cast) would be 0x0000FFFF, not - 1 .