Question

The documentation for WM_NOTIFY says:

• wParam:
  The identifier of the common control sending the message. This identifier is not guaranteed to be unique. An application should use the hwndFrom or idFrom member of the NMHDR structure (passed as the lParam parameter) to identify the control.

And the documentation for NMHDR says:

• idFrom
  An identifier of the control sending the message.

What exactly is the difference between these two?

Answer 1

There is in general no difference. It’s a convenience. The same convenience that you get in the WM_COMMAND message, which passes both an ID and a window handle, even though you can derive the ID from the window handle via GetDlgCtrlID.