window control id

[in] Handle to a menu, or specifies a child-window identifier depending on the window style. For an overlapped or pop-up window, hMenu identifies the menu to be used with the window; it can be NULL if the class menu is to be used. For a child window, hMenu specifies the child-window identifier, an integer value used by a dialog box control to notify its parent about events. The application determines the child-window identifier; it must be unique for all child windows with the same parent window.  MSDN中说it must be unique for all child windows with the same parent window. 如果child window id相同会怎么样呢? ID相同的话,影响的是父窗口接受到notify消息时,id是一样的.区分不了到底是哪个窗口.另外ID如果相同的话,一些notify消息可能不通知父窗口,比如WM_MEASUREITEM消息 This message is sent to the owner window of an owner-drawn button or menu item when the control or menu is created. When the owner window receives the WM_MEASUREITEM message, the owner fills in the MEASUREITEMSTRUCTstructure pointed to by the lParam parameter of the message and returns; this informs the system of the dimensions of the control. If a list box or combo box is created with the LBS_OWNERDRAWVARIABLE or CBS_OWNERDRAWVARIABLE style, this message is sent to the owner for each item in the control; otherwise, this message is sent once.