The return value from DoKeyUp determines whether the control should continue processing the key-up message passed in the Message parameter. A return value of true indicates that the key-up occurrence has been handled completely, and no further processing is needed. A return value of false indicates that the application should continue passing the key-up message through the inherited processing. 

DoKeyUp checks whether the KeyPreview property of the parent form is true, and if so, allows the form to preemptively process key-up messages before they are handled by the windowed control. If the form does not handle the message, DoKeyUp translates the message parameters into the appropriate types and calls KeyUp, which in turn calls the OnKeyUp event handler, if any. 

DoKeyDown 

KeyUp 

OnKeyUp 

KeyPreview