Visual Basic Instinct : Using TypeOf to test interfaces

Using TypeOf to test interfaces

In VB 5.0 and VB 6.0, the new Implements keyword allows you to create objects with multiple interfaces. Sometimes, you might need to test if an object being passed to you really implements the interface you think it does.

Related items

-	Using TypeOf to test interfaces: Link 1

Author: Jens G. Balchen

The solution to this simple problem is TypeOf. Generally used to test if an object has a certain type (read: class), TypeOf will return correct results if used to test against interfaces.

To try this code, create a class called CoolClass. Give it any number of properties and methods you like.


Function IsCoolObject(o As Object) As Boolean

   IsCoolObject = (TypeOf o Is CoolClass)

End Sub

Sub Main()

Dim a As New CoolClass
Dim b As New Collection

   If IsCoolObject(a) Then MsgBox "a is Cool"
   If IsCoolObject(b) Then MsgBox "b is Cool"

End Sub

As you will see, only a is cool. If you create a second class CoolClass2 that Implements CoolClass, you would be able to do this:


Function IsCoolObject(o As Object) As Boolean

   IsCoolObject = (TypeOf o Is CoolClass)

End Sub

Sub Main()

Dim a As New CoolClass2
Dim b As New Collection

   If IsCoolObject(a) Then MsgBox "a is Cool"
   If IsCoolObject(b) Then MsgBox "b is Cool"

End Sub

As you will see, a is still Cool, because a implements CoolClass.