BOOL InSendMessage( )
This function specifies whether the current window function is processing a message that is passed to it through a call to the SendMessage function.
This function has no parameters.
The return value specifies the outcome of the function. It is TRUE if the window function is processing a message sent to it with SendMessage. Otherwise, it is FALSE.
Applications use the InSendMessage function to determine how to handle errors that occur when an inactive window processes messages. For example, if the active window uses SendMessage to send a request for information to another window, the other window cannot become active until it returns control from the SendMessage call. The only method an inactive window has to inform the user of an error is to create a message box.