ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 ENVIRONMENT
If TypeOf pUnk Is ICadLayer Then
Set pCadLayer = pUnk
'do something with the layer
End If
Using the Is operator
If your code requires you to compare two interface reference variables, you can
use the Is operator. Typically, you can use the Is operator in the following circumstances:
• To check if you have a valid interface. For an example, see the following
code:
Enumerators can support other methods, but
these two methods are common among all
enumerators.
Dim pPt As IPoint
Set pPt = New Point
If (Not pPt Is Nothing) Then 'a valid pointer?
... ' do something with pPt
End If
• To check if two interface variables refer to the same actual object. Imagine
that you have two interface variables of type IPoint, pPt1, and pPt2. Are they
pointing to the same object? If they are, then pPt1 Is pPt2.
The Is keyword works with the COM identity of an object. Below is an example
that illustrates the use of the Is keyword when finding out if a certain method on
an interface returns a copy of or a reference to the same real object.
In the following example, the Extent property on a map (IMap) returns a copy,
while the ActiveView property on a MapControl always returns a reference to the
real object.
Dim pEnv1 As IEnvelope
Dim pEnv2 As IEnvelope
Dim pActiveView1 As IActiveView
Dim pActiveView2 As IActiveView
Set pEnv1 = MapControl1.ActiveView.Extent
Set pEnv2 = MapControl1.ActiveView.Extent
Set pActiveView1 = MapControl1.ActiveView
Set pActiveView2 = MapControl1.ActiveView
'Extent returns a copy,
'so pEnv1 is pEnv2 returns false
MsgBox pEnv1 Is pEnv2
'ActiveView returns a reference so,
'ActiveView1 is ActiveView2 returns true
MsgBox pActiveView1 Is pActiveView2
90 • ArcGIS Engine Developer Guide