ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 ENVIRONMENT
Basic has hidden the interface-based programming to simplify the developer
experience. This is not an issue if all the functionality implemented by the object
can be accessed via one interface, but it is an issue when there are multiple interfaces
on an object that provides services.
The Visual Basic Editor backs this up by hiding default interfaces from the
IntelliSense completion list and the object browser. By default, any interfaces that
begin with an underscore, “_”, are not displayed in the object browser (to display
these interfaces, turn Show Hidden Member on, although this will still not display
default interfaces).
You have already learned that the majority of ArcObjects have IUnknown as their
default interface and that Visual Basic does not expose any of IUnknown’s methods,
namely, QueryInterface, AddRef, and Release. Assume you have a class Foo that
supports three interfaces, IUnknown (the default interface), IFoo, and IBar. This
means that if you were to dimension the variable pFoo as below, the variable pFoo
would point to the IUnknown interfaces.
Dim pFoo As New Foo ' Create a new Foo object
pFoo.??????
Since Visual Basic does not allow direct access to the methods of IUnknown, you
would immediately have to QI for an interface with methods on it that you can
call. Because of this, the correct way to dimension a variable that will hold
pointers to interfaces is as follows:
Dim pFoo As IFoo ' Variable will hold pointer to IFoo interface.
Set pFoo = New Foo ' Create Instance of Foo object and QI for IFoo.
Now that you have a pointer to one of the object’s interfaces, it is an easy matter
to request from the object any of its other interfaces.
Dim pBar as IBar ' Dim variable to hold pointer to interface
Set pBar = pFoo ' QI for IBar interface
By convention, most classes have an interface with the same name as the class
with an “I” prefix; this tends to be the interface most commonly used when
working with the object. You are not restricted to which interface you request
when instantiating an object; any supported interface can be requested; hence, the
code below is valid.
Dim pBar as IBar
Set pBar = New Foo ' CoCreate Object
Set pFoo = pBar ' QI for interface
Objects control their own lifetime, which requires clients to call AddRef anytime
an interface pointer is duplicated by assigning it to another variable and to call
Release anytime the interface pointer is no longer required. Ensuring that there are
a matching number of AddRefs and Releases is important, and fortunately, Visual
Basic performs these calls automatically. This ensures that objects do not “leak”.
Even when interface pointers are reused, Visual Basic will correctly call release on
the old interface before assigning the new interface to the variable. The following
code illustrates these concepts; note the reference count on the object at the
various stages of code execution.
Chapter 4 • Developer environments • 83