This method is called during a recording session for any Windows message sent to the control or to one of its children for which the IsKnownPartOf method returns true.

Overridable Sub OnMessage( _
   ByVal src As DependencyObject, _
   ByVal msg As Integer, _
   ByVal wParam As Integer, _
   ByVal lParam As Integer _
) 

virtual void OnMessage( 
   DependencyObject src,
   int msg,
   int wParam,
   int lParam
)

Parameters

src

The control to which the Windows message is targeted. This can be either the control that is served by this custom server, one of its children.

msg

The integer value of the Windows message sent. For example, WM_LBUTTONDOWN is 0x0201.

wParam

The wParam parameter passed with the Windows message.

lParam

The lParam parameter passed with the Windows message.

Reference