What is the usage of WM_NULL?

23 Nov

Posted by hamo as Win32/MFC 1,170 views

Alex Rest found the WM_NULL message and was curious where somebody would need to use that.

I have found a strange Windows message.
It is described in MSDN so:

WM_NULL:
The WM_NULL message performs no operation. An application sends
the WM_NULL message if it wants to post a message that the recipient
window will ignore.

I propose a competition for the the best idea how to use it.

This message really isn’t strange at all. It can be used to verify whether or not an application is still responding. A famous window that does that is the Windows Task Manager by using SendMessageTimeout with WM_NULL. There are a few more methods were you have to use WM_NULL. JohnCz described them very well:

“WM_NULL also is used to filter messages in the WH_GETMESSAGE hook. Returning 0 from a hook does not prevent you from processing a message. Replacing a message with WM_NULL will filter the message properly.”

Another use is an ActiveX control when you decide that it has a toolbar and tooltips. Normally, an application message pump calls PreTranslateMessage; because the ActiveX control does not have one, a message hook can call PreTranslateMessage and, if handled, replace the original message with WM_NULL.