Introduction to COM Part I

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Chapter 2

• Introduction to COM (Part I)

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Objectives

• Describe the COM Architecture
• Discuss Anatomy and benefits of an Interface
• Discuss the standard COM interface
• The IUnknown Interface
• Describe different types of Component Packaging
• In-process Server
• Out-of-process Server
• Remote Server
• Discuss the terminologies associated with COM
• HRESULT
• GUID

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Component Object Model
Elements of COM
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Component Object Model (1)
Advantages

• Fully language-independent

Supports version compatibility
Provides Location Transparency
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Component Object Model (2)
COM does not compete with technologies like DLL
it uses them extensively
Highlights
COM is not a programming language it is a set of
specification
COM is not an API it is a way to develop
component-oriented application
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Component Object Model (3)

• Features
• Creation and use of reusable component objects
• A binary standard for interoperability
• A true system model
• Provides distributed objects

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COM Application Architecture
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Interface
An Interface not only defines how two components,
or a client and a component will talk to each
other but also helps in hiding the business logic
used for developing the object.
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COM Object with Interfaces
Standard Representation
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Using Interfaces With COM Components
Advantages
Reusable application architectures
Hidden internal implementation of the component
Language independence for developing the component
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Attributes of Interfaces

• An Interface is not a class

• An Interface is not a component object

• Interfaces are immutable

• Clients interact only with the pointers to the
  Interfaces

• Components can implement multiple interfaces

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IUnknown Interface
The IUnknown interface helps the client
application to scan for other interfaces
available in the component and get their
pointers.
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Component Packaging Types
In-process server
  Out-of-process server
Remote server
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In-Process Server
In-proc Object
In-proc Server
Client Application
Client Process
The in-proc servers are implemented in the form
of a DLL.
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In-process Server (1)
Advantages
Saves memory space
Saves disk space
Upgradations are easy
Supports multiple languages
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Out-of-Process Server
The out-of process servers are implemented in the
form of an EXE.
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Out-of-Process Server (1)
Advantages
Crash protection
Support for User Interface
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Out-of-Process server (2)
Drawbacks
Slow responsiveness
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Remote Server
Remote Machine
Client Machine
The remote servers are implemented in the form of
either EXE or DLL.
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Remote Server (1)
Advantages
A single component can be shared by multiple
client applications running on multiple systems
The components can be upgraded very easily
It forms the basis for component based
distributed client/server architecture
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Remote Server (2)
Drawbacks
Slow responsiveness
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HRESULT
The call may or may not be successful
Processing is carried out further depending on
the HRESULT value
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Globally Unique Identifiers
Just as a vehicle or a passport have unique
numbers for identification, components also have
unique ids called Globally Unique IDentifiers
(GUID)