ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 ENVIRONMENT
While Wend constructs
Avoid While … Wend constructs. Use the Do While … Loop or Do Until ... Loop
instead because you can conditionally branch out of this construct.
pFoos.Reset
Set pFoo = pFoos.Next
Do While (Not pFoo Is Nothing)
If (pFoo.Answer = "Done") Then Exit Loop
Set pFoo = pFoos.Next
Loop
The VBVM was called the VB Runtime in earlier
versions of the software.
The Visual Basic Virtual Machine
The Visual Basic Virtual Machine (VBVM) contains the intrinsic Visual Basic
controls and services, such as starting and ending a Visual Basic application,
required to successfully execute all Visual Basic developed code.
The VBVM is packaged as a DLL that must be installed on any machine wanting
to execute code written with Visual Basic, even if the code has been compiled to
native code. If the dependencies of any Visual Basic compiled file are viewed,
the file msvbvm60.dll is listed; this is the DLL housing the Virtual Machine.
For more information on the services provided by the VBVM, see the sections
‘Interacting with the IUnknown interface’ and ‘Working with HRESULTs’ in this
chapter.
Interacting with the IUnknown interface
The section ‘The Microsoft Component Object Model’ earlier in this chapter
contains a lengthy section on the IUnknown interface and how it forms the basis
on which all of COM is built. Visual Basic hides this interface from developers
and performs the required interactions (QueryInterface, AddRef, and Release function
calls) on the developer’s behalf. It achieves this because of functionality
contained within the VBVM. This simplifies development with COM for many
developers, but to work successfully with ArcObjects, you must understand what
the VBVM is doing.
Visual Basic developers are accustomed to dimensioning variables as follows:
Dim pColn as New Collection ' Create a new collection object.
PColn.Add "Foo", "Bar" ' Add element to collection.
It is worth considering what is happening at this point. From a quick inspection
of the code, it appears that the first line creates a collection object and gives the
developer a handle on that object in the form of pColn. The developer then calls a
method on the object Add. Earlier in the chapter you learned that objects talk via
their interfaces, never through a direct handle on the object itself. Remember,
objects expose their services via their interfaces. If this is true, something isn’t
adding up.
What is actually happening is some “VB magic” performed by the VBVM and
some trickery by the Visual Basic Editor (VBE) in the way that it presents objects
and interfaces. The first line of code instantiates an instance of the collection
class, then assigns the default interface for that object, _Collection, to the variable
pColn. It is this interface, _Collection, that has the methods defined on it. Visual
82 • ArcGIS Engine Developer Guide