Microsoft COM

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Microsoft COM

• Component Object Model

Ashish Kasturia ashish_at_yakko.cs.wmich.edu
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Agenda

• Introduction
• Overview
• The Component Software Problem
• COM features
• COM standard comes to the rescue
• Client/Server model

• Inheritance and reusability
• OLE, ActiveX, DCOM
• Samples
• Miscellaneous
• Summary
• References
• Questions

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Introduction

• The history of software development
• The beginners way - procedural way
• Object oriented way
• The DLL hell problem.
• The COM way of developing software.
• Code reuse vs. object reuse

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Overview

• for(i 0 i lt 100 i)
• printf(Hello World!)

printf(Goodbye World!)
gcc .cpp compiling 1000 files ...
gcc .cpp compiling 1000 files ...
Why do you have to recompile the entire project
when you change a small module?
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The Component Software Problem

• Interoperability
• Making components from independent vendors work
  with each other.
• Version problem
• Upgrading the component, without recompiling the
  entire project.
• Language independence
• Platform independence

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COM features

• A binary standard between components.
• Grouping of functions into Interfaces
• Uniquely identify Interfaces and components.
• Ways for components to dynamically discover
  Interfaces of other components.
• Ways for components to track their own lifetimes.

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COM features

• A binary standard between components.
• COM defines a standard way to lay out the virtual
  function tables (vtables) and a standard way to
  call functions.

Client variable
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COM features

• Grouping of functions into Interfaces
• An interface is a strongly typed contract between
  components to provide a small and useful set of
  semantically related methods.
• The interaction between components is done only
  through interfaces.
• So, only the methods can be called, but data
  cannot be modified.

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COM features

• More about Interfaces
• Interface is a set of related methods in a
  component.
• Components can have multiple interfaces.
• Interfaces are strongly typed.
• Every interface is uniquely identified by a GUID.
• Interfaces are immutable.

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COM features

• More about Interfaces

In this case, the object supports 3 interfaces -
A, B and Iunknown. Client uses Interface A.
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COM features

• Uniquely identify Interfaces and components.
• Global Unique Identifiers (GUIDs) are 128 bit
  integers and are used to identify the interfaces
  and component objects.
• GUIDs are UUIDs (universally unique IDs) as
  defined by the Open Software Foundation's
  Distributed Computing Environment.

E.g. IUnknown - 00000000-0000-0000-C000-000000000
046 IIDs found under HKEY_CLASSES_ROOT\Interface
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COM features

• Ways for components to dynamically discover
  Interfaces of other components.
• All the interfaces have to be derived from
  IUnknown interface.
• IUnknown interface has three methods called
  QueryInterface, Addref and Release.
• QueryInterface method is used to query for other
  interfaces.

hr ppB-gtQueryInterface(IID iidA,void
ppA) hr ppA-gtMethodA() hr
ppA-gtQueryInterface(IID iidB,void ppB)
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COM features

• Ways for components to track their own lifetimes.
• Done via Addref and Release methods.
• Whenever a client uses an interface, Addref is
  called. When the client no longer requires the
  interface, Release is called.
• In this way, a reference count is maintained by
  the object.

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COM standard comes to the rescue

• Interoperability
• Standard use of vtables.
• Versioning.
• Each component can be updated without any effect
  on the rest of the application.
• Language independence
• COM represents a binary standard, as opposed to
  source code standard.
• Platform and local/remote transparency.

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Client/Server model

• The components behave as servers, serving
  interfaces to clients.
• If a client requests an interface that isnt
  supported by a server, E_NOTIMPL is returned and
  the client handles the situation accordingly.
• If a server crashes, the client can gracefully
  handle the situation.

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Inheritance and reusability

• Implementation inheritance in component
  development is not a good idea.
• COM provides containment/aggregation.

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OLE, ActiveX, DCOM

• Automation as defined by MSDN A way to
  manipulate an application's objects from outside
  the application. E.g OLE, ActiveX.
• It uses IDispatch interface to get the methods
  and the object properties. In/out variables
  specified in IDL.
• DCOM it is COM with a longer wire, uses proxy
  and stub interfaces and works on remote servers
  and clients.

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Samples

• Making a simple COM component in VC.
• Adding automation capabilities.
• Using the object in VB.
• Using the object in VC.
• Using the regsvr32 utility

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Miscellaneous

• Security
• Communications between components, even across
  process and network boundaries.
• Shared memory management between components
• Error and status reporting - HRESULT
• How does VB use it?
• COM on Linux/Unix?

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Summary

• COM solves the Component Software problem.
• Imposes standards on the way components interact
  with each other (same process, different
  processes or remote components).
• Allows the components can be built by independent
  vendors.
• Allows for upgrading the components without any
  effect on other components.

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References

• MSDN Library Visual Studio 6.0 release
• The Component Object Model A Technical Overview.
• COMponents.
• Dr. GUIs articles.
• Microsofts COM website
• IDevResource.com COM channel - COM on Linux by
  Frank Rem
• IDevResource.com - What COM is all about

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Question?