ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 DEVELOPMENT ENVIRONMENT
The previous section of this chapter focused primarily on how to write code in
the VBA development environment embedded within the ArcGIS Desktop
applications. This section focuses on particular issues related to creating ActiveX
DLLs that can be added to the applications and writing external standalone
applications using the Visual Basic development environment.
The ESRI VB Add-In interface implementer can
be used to automate Steps 3 and 4.
CREATING COM COMPONENTS
Most developers use Visual Basic to create a COM component that works with
ArcMap or ArcCatalog. Earlier in this chapter you learned that since the ESRI
applications are COM clients—their architecture supports the use of software
components that adhere to the COM specification—you can build components
with different languages including Visual Basic. These components can then be
added to the applications easily. For information about packaging and deploying
COM components that you’ve built with Visual Basic, see Chapter 5, ‘Licensing
and deployment’, in this guide.
This section is not intended as a Visual Basic tutorial; rather, it highlights aspects
of Visual Basic that you should know to be effective when working with
ArcObjects.
In Visual Basic you can build a COM component that will work with ArcMap or
ArcCatalog by creating an ActiveX DLL. This section will review the rudimentary
steps involved. Note that these steps are not all-inclusive. Your project may
involve other requirements.
1. Start Visual Basic. In the New Project dialog box, create an ActiveX DLL
Project.
2. In the Properties window, make sure that the Instancing property for the
initial class module and any other class modules you add to the Project are set
to 5—MultiUse.
3. Reference the ESRI object libraries that you will require.
4. Implement the required interfaces. When you implement an interface in a class
module, the class provides its own versions of all the public procedures specified
in the type library of the interface. In addition to providing mapping
between the interface prototypes and your procedures, the Implements statement
causes the class to accept COM QueryInterface calls for the specified
interface ID. You must include all the public procedures involved. A missing
member in an implementation of an interface or class causes an error. If you
don’t put code in one of the procedures in a class you are implementing, you
can raise the appropriate error (Const E_NOTIMPL = &H80004001). That
way, if someone else uses the class, they’ll understand that a member is not
implemented.
5. Add any code that’s needed.
6. Establish the Project Name and other properties to identify the component. In
the Project Properties dialog box, the project name you specify will be used as
the name of the component’s type library. It can be combined with the name
of each class the component provides to produce unique class names (these
names are also called ProgIDs). These names appear in the Component Category
Manager. Save the project.
92 • ArcGIS Engine Developer Guide