Canceling an event

Microsoft Outlook calls event handlers in your program to allow your program to respond to such events as actions that the user takes or changes in the message store. Each event is accompanied by a default action that Outlook performs as a result of the event. For example, when the Open event occurs for an item, by default Outlook displays the item in an inspector window.

Some events only notify your program that a particular event has occurred. For these events, your event handler simply responds to the event. With other events, Outlook allows your event handler to cancel the event, that is, to instruct Outlook not to perform the default action associated with the event. In the case of the Open event, for example, your program can prevent Outlook from displaying the item in an inspector. If an event can be cancelled, the reference topic describing the event indicates how to cancel the event.

If an event can be cancelled, an event handler written in Microsoft Visual Basic or Microsoft Visual Basic for Applications receives a parameter that it sets before returning to indicate whether the event should be cancelled. For example, an event handler for the Open event written in Visual Basic for Applications might look like this. This example assumes that the value of OpenOK is set elsewhere.

Sub myItem_Open(byRef Cancel as Boolean)
    If OpenOK Then
        Cancel = False ' Outlook performs default action
    Else
        Cancel = True  ' Outlook does not perform default action
    EndIf
End Sub

Because of limitations in VBScript, however, this syntax cannot be used. An event handler for the Open event in the script of an item must be written as a function. To cancel the event, the value of the function is set to False before returning, as in the following example.

Function Item_Open()
    If OpenOK Then
        Item_Open = True  ' Outlook performs default action
    Else
        Item_Open = False ' Outlook does not perform default action
    End If
End Function