A more efficient way to test Visual Basic class interfaces

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　　A more efficient way to test Visual Basic class interfaces

If you develop applications that use several versions of a component,
you might have read "Polymorphism, Interfaces, Type Libraries, and

GUIDs" in VB注释：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
注释： (Error handling code omitted.)
注释： Attempt to access the preferred interface.
Set ifin2 = fnr
If Err.Number <> 0 ThenIf Err.Number <> 0 Then
   注释： Access the more limited interface.
   Set ifin = fnr
   注释： (Code to provide limited functionality,
   注释：   using the object variable ifin.)
Else
   注释： (Code to provide full functionality,
   注释：   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
注释： (Errors produced by previous line are handled
注释：   in YourErrorHandler).

注释： Check to see if fnr supports the IFinance2 interface
If Not TypeOf fnr Is IFinance2 Then
   注释： Access the more limited interface.
   Set ifin = fnr
   注释： (Code to provide limited functionality,
   注释：  using the object variable ifin.)
Else
   set ifin2 = fnr
   注释： (Code to provide full functionality,
   注释：   using the object variable ifin2.)
End If

Note that you can注释：t use

If TypeName(fnr) <> "IFinance2" Then

because TypeName() always returns the actual class name of the
instantiated object, not the interfaces it implements.

This tip was contributed by Travis E. Hyatt.

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