ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 ENVIRONMENT
Private Sub VBMagic()
' Dim a variable to the IUnknown interface on the simple object.
Dim pUnk As IUnknown
See the Visual Basic Magic sample on the disk
for this code. You are encouraged to run the
sample and use the code. This object also uses
an ATL C++ project to define the SimpleObject
and its interfaces; you are encouraged to look at
this code to learn a simple implementation of a
C++ ATL object.
' Co Create simpleobject asking for the IUnknown interface.
Set pUnk = New SimpleObject 'refCount = 1
' QI for a useful interface.
' Define the interface.
Dim pMagic As ISimpleObject
' Perform the QI operation.
Set pMagic = punk 'refCount = 2
' Dim another variable to hold another interface on the object.
Dim pMagic2 As IAnotherInterface
' QI for that interface.
Set pMagic2 = pMagic 'refCount = 3
' Release the interface pointer.
Set pMagic2 = Nothing 'refCount = 2
' Release the interface.
Set pMagic = Nothing 'refCount = 1
' Now reuse the pUnk variable - what will VB do for this?
Set pUnk = New SimpleObject 'refCount = 1, then 0, then 1
' Let the interface variable go out of scope and let VB tidy up.
End Sub 'refCount = 0
Often interfaces have properties that are actually pointers to other interfaces.
Visual Basic allows you to access these properties in a shorthand fashion by
chaining interfaces together. For instance, assume that you have a pointer to the
IFoo interface, and that interface has a property called Gak that is an IGak interface
with the method DoSomething. You have a choice on how to access the
DoSomething method. The first method is the long-handed way.
Dim pGak as IGak
Set pGak = pFoo ' Assign IGak interface to local variable.
pGak.DoSomething ' Call method on IGak interface.
Alternatively, you can chain the interfaces and accomplish the same thing on one
line of code.
pFoo.Gak.DoSomething ' Call method on IGak interface.
When looking at the sample code, you will see both methods. Normally, the
former method is used on the simpler samples, as it explicitly tells you what
interfaces are being worked with. More complex samples use the shorthand
method.
84 • ArcGIS Engine Developer Guide