Just because you write case WM_SOMETHING: doesn’t mean that you have to handle all possible parameters for the WM_SOMETHING message. You’re still allowed to call the DefWindowProc function. After all, that’s what you did when you didn’t have a case WM_SOMETHING: statement in the first place.

switch (uMsg) {
case WM_CHAR:
    OnChar(…);
    return 0;

default:
    return DefWindowProc(…);
}

The above code fragment doesn’t handle the WM_SOMETHING message at all. Suppose the WM_SOMETHING message uses the wParam parameter to specify what type of something occurred, and you only want to override the default processing in the case where wParam has the value of 4. What do you do with the other values?

switch (uMsg) {
case WM_CHAR:
    OnChar(…);
    return 0;

case WM_SOMETHING:
    if (wParam == 4) { DoSomething4(…); }
    else … ????? …
    return 0;
default:
    return DefWindowProc(…);
}

If the value is 4, then you do your special “something 4” processing, but what about all the other values? How do you handle them?

Well, think about it: How did you handle them before? The original code, before you added a WM_SOMETHING handler, was equivalent to this:

switch (uMsg) {
case WM_CHAR:
    OnChar(…);
    return 0;

case WM_SOMETHING:
    return DefWindowProc(…);
default:
    return DefWindowProc(…);
}

In the original code, since there was no explicit handler for the WM_SOMETHING message, control is transferred to the default case handler, which just calls the DefWindowProc function. If you really want to, you can expand the case out a bit more:

switch (uMsg) {
case WM_CHAR:
    OnChar(…);
    return 0;

case WM_SOMETHING:
    if (wParam == 4) return DefWindowProc(…);
    else return DefWindowProc(…);
default:
    return DefWindowProc(…);
}

Because if the wParam is 4, the original code just called DefWindowProc. And if the wParam was something other than 4, the original code still just called DefWindowProc.

Of course, I expanded the block in precisely this way so it matches up with the case we started writing when we decided to handle the WM_SOMETHING method. Written out this way, it becomes obvious what to write for the question marks.

switch (uMsg) {
case WM_CHAR:
    OnChar(…);
    return 0;

case WM_SOMETHING:
    if (wParam == 4) { DoSomething4(…); }
    else return DefWindowProc(…);
    return 0;
default:
    return DefWindowProc(…);
}

Just because you have a case WM_SOMETHING statement doesn’t mean you have to handle all the cases; you can still call DefWindowProc for the cases you don’t want to handle.

Armed with this information, you can help commenter Norman Diamond handle the VK_F10 key in his WM_SYSKEYDOWN message handler without having to “start handling a bunch of keys that really are system keys, that I didn’t want to bother with.”