Subclassing and Hooking with Visual Basic

Intercepting Keystrokes in a WinForm

I want to cover one final method of intercepting messages in a WinForm. There are nine built-in methods in the Control class that you can override to capture keystroke messages. If you need to capture only keyboard messages, using one or more of these methods will allow you to intercept the messages without having to subclass the WinForm.

The messages that you can intercept are:

WM_CHAR

WM_KEYDOWN

WM_KEYUP

WM_SYSKEYDOWN

WM_SYSKEYUP

The nine methods that you can override are:

ProcessKeyPreview

A child control calls this method before processing a keyboard message. If the child control does not process the message, it is sent on to the parent control. If no parent control exists, this method returns True, indicating that this message is not to be processed any longer.

PreProcessMessage

This method is called from the thread’s message loop, before the message is dispatched. The processing that this method performs depends on the message received:

WM_KEYDOWN and WM_SYSKEYDOWN

ProcessCmdKey is first called to determine if the keyboard message is an accelerator or menu shortcut. If ProcessCmdKey returns False, IsInputKey is called to determine if the keyboard message is being sent as input to the control, such as characters being typed into a text box control. Finally, if IsInputKey returns False, the keyboard message is sent to ProcessDialogKey. ProcessDialogKey determines if the key is a dialog type key such as a TAB, an arrow, or mnemonics. ...