If you develop applications that use several versions of a component,
you might have read "Polymorphism, Interfaces, Type Libraries, and
GUIDs" in VB's Help file. Microsoft gives an example of how to test
whether an object supports a particular interface:
Dim fnr As FinanceRules
Dim ifin As IFinance
Dim ifin2 As IFinance2
On Error Resume Next
Set fnr = New FinanceRules
' (Error handling code omitted.)
' Attempt to access the preferred interface.
Set ifin2 = fnr
If Err.Number <> 0 ThenIf Err.Number <> 0 Then
' Access the more limited interface.
Set ifin = fnr
' (Code to provide limited functionality,
' using the object variable ifin.)
Else
' (Code to provide full functionality,
' using the object variable ifin2.)
End If
As you can see, this example uses inline error handling to determine
whether a type-mismatch error occurs when setting ifin2 = fnr. If your
routine already has an active error handler, your code can become
cluttered with this solution--especially if you test for several
different interfaces in a single routine.
To make your code more readable, use:
On Error Goto YourErrorHandler
Set fnr = New FinanceRules
' (Errors produced by previous line are handled
' in YourErrorHandler).
' Check to see if fnr supports the IFinance2 interface
If Not TypeOf fnr Is IFinance2 Then
' Access the more limited interface.
Set ifin = fnr
' (Code to provide limited functionality,
' using the object variable ifin.)
Else
set ifin2 = fnr
' (Code to provide full functionality,
' using the object variable ifin2.)
End If
Note that you can't use
If TypeName(fnr) <> "IFinance2" Then
because TypeName() always returns the actual class name of the
instantiated object, not the interfaces it implements.