Creating COM DLLs with Visual Basic

1
Chapter 3

• Creating COM DLLs with Visual Basic
• Lesson 1 Creating COM DLLs and EXEs

2
Learning Objectives

• Set project properties for COM DLLs.
• Describe the different threading models for
  Visual Basic components.
• Create, test, and debug COM components.

3
Introduction

• In lesson 2, you learned how to create simple
  class modules from which you can create objects.
  In this lesson, you will go a step further by
  implementing class modules with COM code
  components.
• Code components provide libraries of programmable
  objects because they contain one or more class
  modules.

4
COM DLLs vs COM EXEs

• COM DLLs typically run in-process, allowing
  faster access to objects but are less
  fault-tolerant. If the DLL fails, the entire host
  process fails.
• And COM EXEs run out-of-process, providing better
  fault tolerance because faults are limited to
  just the out-of-process EXE. If the EXE fails,
  other processes in the system will not fail.
  However, COM EXEs are always slower than COM DLLs
  because method calls must be marshaled between
  processes.

5
Threading Model

• A process is an instance of an application that
  has an address space with four gigabytes of
  virtual memory and other resources.
• Threads are objects that live in a process, and
  are the actual pieces of code that are executed
  on the processor.
• Each process starts with one thread, called the
  primary thread. A process always has one or more
  threads executing.

6
Apartment Threaded

• The apartment threaded is a COM-specific form of
  multithreading. It is essentially a way of
  describing a thread with a message queue that
  supports COM objects. All COM components created
  with VB use apartment model threading.

7
Using a Project Group to Test a COM DLL

• To create a COM DLL in VB
• To create a test project

8
Lab Creating a COM DLL