Intro to COM

1
Intro to COM

• What is it and how do I use it?

2
Objectives

• Teach the fundamentals of COM.
• Understand the reason for using it.
• Learn to make a simple in-process server.

3
COM Defined

• COM (Component Object Model) and DCOM
  (Distributed COM)
• are a set of binary and network standards for the
  modeling of objects and their communications.

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What does it do?

• It allows for Plug and Play software.
• Helps to create distributed systems transparently.

5
Why should you learn COM?

• Flexibility can be used from and written in all
  popular languages.
• Saves tremendous time (after the initial learning
  curve)
• Use of existing objects
• Object Reuse C tried to address this issue,
  but just made it easy to reuse source code.
• So you can use DCOM.

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Why should you learn COM?

• Rapid Application Development
• Seamless application updates
• Build great apps without reinventing the wheel
• Interface different systems with ease

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Component Hierarchy

• Components are made up of objects
• Objects are made up of interfaces
• Interfaces are made up of member functions

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What makes an object?

• Objects support object-oriented principles
• encapsulation
• polymorphism
• inheritance?
• Conceptual in nature
• Known only through interfaces

Object
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What makes an object?

• Clients can interact with any object without
• knowing which object with which it interacts

Object 1
Client
Object 2
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The Specifics
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Identifiers

• GUID (globally unique identifiers)
• are stored in 16-byte (128 bit) structures
• CLSID (class identifier GUID)
• IID (interface identifier GUID)

IUnknown

• QueryInterface
• Returns a pointer to requested Interface
• AddRef
• Increases the RefCount
• Release
• Decreases the RefCount

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Interfaces

• An interface is a set of similar methods
• Analogous to a C class
• Binary level, not source code level
• Indentified by an Interface Identifier (IID)

interface ISquirrel IUnknown HRESULT
EatNuts() HRESULT ClimbTree() HRESULT
RunAway()
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IUnknown

• Base interface
• Polymorhic to all other interfaces
• Implements reference counting
• All other interfaces are retrieved through
  IUnknown

interface IUnknown ULONG AddRef() ULONG
Release() HRESULT QueryInterface(REFIID iid,
void ppObject)
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Accessing Objects
STDAPI CoCreateInstance( REFCLSID
rlcsid, LPUNKNOWN pUnkOuter, DWORD
dwClsContext, REFIID riid, LPVOID ppv )

• CoCreateInstance
• Based on a class ID (CLSID)
• Specify requested interface
• Object can be anywhere

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Where do components live?

• In-Process servers (.dll)
• Out-of-Process servers (.exe)
• Remote servers (.exe)
• ActiveX (.ocx)

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Uses

• ActiveX
• DCOM
• DirectX
• Automation
• OLE
• etc.

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Recommended Reading

• The definitive beginners guide to COM
• Inside COM by Dale Rogerson
• A great online tutorial that summarizes much of
  Inside COM
• http//beta.develop.com/comtutorial
• For the complete history and theory
• Inside OLE by Kraig Brockschmidt