When creating any window, the system sends messages to the window procedure for the window. The system sends the WM_NCCREATE message after creating the window's nonclient area and the WM_CREATE message after creating the client area. The window procedure receives both messages before the system displays the window. Both messages include a pointer to a CREATESTRUCT structure that contains all the information specified in the CreateWindowEx function. Typically, the window procedure performs initialization tasks upon receiving these messages.
When creating a child window, the system sends the WM_PARENTNOTIFY message to the parent window after sending the WM_NCCREATE and WM_CREATE messages. It also sends other messages while creating a window. The number and order of these messages depend on the window class and style and on the function used to create the window. These messages are described in other topics in this help file.