winapi - Windows API: What is the first message a window is guaranteed to receive?

Question

I've been used to thinking that WM_CREATE is the first message a window receives. However, when testing this assumption on a top-level window, it turns out to be false. In my test, WM_MINMAXINFO turned up as the first message.

So, what is the first message a window is guaranteed to receive?

Answer 1

WM_NCCREATE is actually the very first message your window will receive, which will arrive before WM_CREATE. It is related to creating the non-client area (eg. title bar, system menu, etc), hence the NC prefix.

WM_GETMINMAXINFO is sent before the window size/position is changed, and may arrive before WM_CREATE (see below for more).

The WM_CREATE message is sent before CreateWindow() returns, so you can guarantee that per-window initialisation has been performed by that point. Your window proc will receive WM_CREATE after the window is created, but before the window becomes visible (WM_SHOWWINDOW).

Actually, there is an interesting inconsistency in the MSDN documentation - the creation messages seem to depend on whether you call CreateWindow() or CreateWindowEx(), however it does not specify that the messages are necessarily listed in order of dispatching.

• CreateWindow(): WM_CREATE, WM_GETMINMAXINFO and WM_NCCREATE
• CreateWindowEx(): WM_NCCREATE, WM_NCCALCSIZE, and WM_CREATE

I strongly suspect that the message order described in CreateWindow() should have WM_NCCREATE first, and the regular WM_CREATE last, which is consistent with the notification documentation and the CreateWindowEx() reference (and also consistent with what you describe).

Raymond Chen also has some interesting information on window creation/destruction.

It just goes to show, even seemingly simple things can get complex the more you look at them.