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.
Remarks
For example, if an application has installed a WH_GETMESSAGE hook and wants to prevent a message from being processed, the GetMsgProc callback function can change the message number to WM_NULL so the recipient will ignore it.
As another example, an application can check if a window is responding to messages by sending the WM_NULL message with the SendMessageTimeout function.
