COM A Slideshow of Epic Proportions

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COMA Slideshow of Epic Proportions

• Sean Baxter
• http//www.eburg.com/baxters/

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What is COM?

• A standard for interoperable software
• A runtime library providing low-level or
  hard-to-implement functionality
• A competitive, capitalist marketplace for selling
  functionality to application developers
• An endless source of healthy obfuscation

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That Last Slide Was Pulled From Booboos Rear
End. What is COM, Really?

• A sacred covenant held by the worlds best
  developers. All software functionality is
  encapsulated through a COM class (coclass) and
  exposed through COM interfaces

Mooooo
Blah Blah Blah Blah Blah Blah Blah Blah Blah
Booboo
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Interface and Coclass Fundamentals

• An interface is a group of methods
• A coclass is a group of interfaces
• All coclasses implement the IUnknown interface
• All other interfaces derive IUnknown
• Lollipop diagrams describe coclasses

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About Interfaces

• The client communicates with the server through
  interface pointers (think base class pointers)
• Methods implemented by the coclass are invoked
  through interface pointers
• Interface pointers are acquired through IUnknown
• Because all interfaces derive IUnknown, any
  interface pointer on an object can be acquired
  through any other interface pointer on the object
• The method to acquire an interface pointer is
  IUnknownQueryInterface (think dynamic_cast)

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More About Interfaces

• Interfaces are equivalent to C abstract base
  classes
• Because there is no standard thunk mechanism,
  interfaces do not support multiple inheritance
• Two IUnknown pointers point to the same object if
  their values are equal
• Dispatch interfaces are special case interfaces
  allowed by the COM Specification

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Interfaces and Virtual Tables

• An interface pointer points to the corresponding
  slice of a COM object
• The first 4 bytes of this slice comprise a
  pointer to the interfaces virtual table
• Each slot in this virtual table is the address of
  an interface method
• A method implementation may be referenced by more
  than one vtbl entry

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Interface Method Indirection
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Reference Counting

• To regulate destruction of objects, COM requires
  that all coclasses implement a reference counter
• COM objects can have their methods invoked from
  other threads, processes, and even machines.
  Reference Counting eliminates a number of
  problems associated with synchronization

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Reference Counting Rules

• Each object is given a reference count of 1 upon
  instantiation
• IUnknownAddRef increments the reference count
• A successful IUnknownQueryInterface call
  increments the reference count
• IUnknownRelease decrements the reference count
• When the reference count is decremented to zero,
  the object destroys itself

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So Why All These Interface and Reference Counting
Formalities?

• One of the most amazing features of COM objects
  is location transparency COM marshals interfaces
  between processes and remote computers
• Reference counting guarantees that a client
  cannot prematurely destroy a shared object
• COM is language transparent unsophisticated
  languages like Visual Basic, VBA, J-Script, and
  VB-Script would not otherwise be able to properly
  delete objects

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Speaking of Location Transparency...

• COM objects (servers) can be delivered in two
  module types EXEs or DLLs (aka OCXs)
• EXEs cannot run in-process
• DLLs can only run in-process
• A surrogate EXE can load a DLL to make it an
  out-of-process server

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An In-process Server (same apartment)
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In-process Server(different apartment)
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Local Server
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Remote Server
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Things To Note

• All interface method calls are synchronous a
  remote call to a New York server must complete
  its round-trip before the client can continue
  execution
• Asynchronous services are available through
  Microsoft Message Queue (MSMQ) and ICallFactory
  (Windows 2000)
• Encryption, authentication, and impersonation
  services are available with remote server
  connections (at least on NT)

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Confused?
Where Am I?
The Guy In The Back Row
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COM Owns You

• So what does all this nonsense about location
  transparency and reference counting have to do
  with anything?
• Ever heard of OLE? Ever heard of ActiveX? You
  may suspect that these have something to do with
  COM...

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WRONG!

• They are COM.

All will soon be clear
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MS MarketectureCommon Misperceptions

• OLE is a system of linking and embedding data
  into a document (hence the acronym Object Link
  and Embedding)
• ActiveX, like Java, is a system for building web
  page applets
• Microsoft Transaction Server (MTS) processes
  business data from the internet

Faery Tales!
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The Journey of COM

• Dynamic Data Exchange (DDE) was introduced while
  the Earth was still cooling.
• DDE was unsavory. MS unveils OLE 1.0 after the
  villagers riot
• OLE 1.0 is a big success but hard to program. MS
  unveils OLE 2.0 for Windows 3.1 COM is born
  (its 16bit)
• Windows NT 3.5 is blessed with OLE (look ma - no
  version number!) and true 32bit COM. OLE is now
  Olè, which is synonymous with COM

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The Journey of COM (II)

• To celebrate Windows 95, MS blesses COM with the
  apartment model
• 1996 sees Windows NT 4. DCOM brings network and
  multithreading enhancements to COM
• Microsoft Transaction Server (MTS) becomes
  available for NT Server. This doesnt process
  web business data. Its a COM runtime
  environment which allows components to run more
  efficiently (solving scalability difficulties
  often faced in processing business data)

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The Journey of COM (III)

• OLE-DB provides consistency when accessing data
  from a variety of providers
• MS makes web browsers a priority (IE3) and
  simultaneously coins the term ActiveX, adding
  to the COM marketecture. Like OLE, ActiveX is a
  synonym for COM.
• Microsoft Message Queue (MSMQ) delivers
  asynchronous (and vastly more reliable) method
  calls
• Visual C 5.0 ships with ATL 2.1. OLE Control
  creation has never been so easy!

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The Journey of COM (IV)

• Windows 2000 ships in 2004. Microsoft introduces
  COM acronym COM is simply COM on Win2k
• Win2k includes many COM updates including a third
  threading model (the Neutral Threaded Apartment)
  and integration of MTS into the COM runtimes
• COM 2.0 delivers legendary COM virtual machine
• Windows NT 6 device contexts replaced by
  DirectDraw surfaces. Classic OLE Control
  interfaces deprecated for new, ultra-spiffy
  interfaces

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COMprehension

• COM is a set of many, many technologies.
• COM is really Microsofts software engineering
  strategy
• Computer people (you) are betting the bank on
  Microsoft
• Microsoft is betting the bank on COM
• COM Owns You

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COM Defined

• A single sentence cant define COM, because COM
  is such a monumental and omnipresent set of
  technologies.
• So what is a COM technology? Any software
  creation that leverages interfaces and coclasses,
  as described in the earlier slides.

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COM in Everyday Life
The Month View is from the Microsoft Windows
Common Controls-2 6.0 controls library
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Property Pages
How does Visual Basic know which properties to
put in the box?
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Events
How does Visual Basic know which events the
control exposes?
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What is a VB Control?
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What Functionality Does It Expose?
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Properties and Methods
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The Secret of Property Pages
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Property Page Coclasses
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Property Pages Abound!
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Standard Interfaces

• Programming sucks. Best to fall back on
  platform-provided functionality
• A container/client can interact with any
  server/control simply by implementing the
  required site interfaces
• Application developers who license your
  components are already familiar with the
  interfaces
• Standard interfaces can use the
  Microsoft-provided proxy/stubs

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What Are Some COM-Based Technologies?

• OLE Controls
• OLE-DB
• ADO
• DAO
• DirectX
• TAPI 3.0
• Exchange Server
• Exchange Client
• Active Scripting

• CDO 2.0
• MAPI
• SQL-DMO
• SQL-NS
• Microsoft Repository
• Broadcast Architecture
• Still Image API
• Microsoft Multimedia
• Indexing Service

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Still More COM Technologies!

• Active Desktop
• IIS
• Microsoft Agent
• Microsoft Java SDK
• NetMeeting
• NetShow
• Microsoft Wallet
• Active Directory
• Active Accessibility
• Clustering Service

• Fax Services
• MSMQ
• RAS
• Synchronization Manager
• Structured Storage
• MTS
• MMC
• Windows Shell API
• all OLE Applications

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The Glory of COM

• COM eliminates thread synchronization and IPC/RPC
  confusion
• COM enables real polymorphism language
  transparency and location transparency
• COM mandates a consistency among component
  interfaces to reduce learning curves
• COM allows versionless updates
• COM is the best component standard it is by far
  the most commercially successful

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DLL Server Mechanisms

• DllRegisterServer and DllUnregisterServer
  manipulate the registry
• DllCanUnloadNow tells COM when the module is not
  being used
• DllGetClassObject provides the client with the
  class factory of the required COM object

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EXE Server Mechanisms

• A /RegServer argument registers the EXE
• A /UnregServer argument un-registers the EXE
• CoRegisterClassObject registers a class factory
  for clients to connect to
• RegisterActiveObject adds an object to the
  Running Object Table (ROT)

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COM Object Instantiation

• CoGetClassObject (wrapped by CoCreateInstance and
  CoCreateInstanceEx) launches a module and
  retrieves its class factory
• CoGetClassObjectFromURL downloads a module from
  an URL, decompresses it, registers it, and
  invokes CoGetClassObject on it
• GetActiveObject loads an object off the ROT

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COM Security

• CoInitializeSecurity sets the authentication
  requirements for the current process
• The COSERVERINFO structure specifies the machine
  name and authentication info for clients
  connecting to remote objects
• CoImpersonateClient allows the server to
  impersonate the client when accessing other
  servers

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Books of COMpetency

• The C Programming Language (Bjarne Stroustrup)
• Programming Windows (Charles Petzold)
• Advanced Windows (Jeffrey Richter)
• Design Patterns (Gamma, Helm, Johnson, Vlissides)
• Essential COM (Don Box)

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Hoorah For Reading!

• Effective COM (Box, Brown, Ewald, Sells)
• Inside OLE (Kraig Brockschmidt)
• Professional ATL COM Programming (Richard Grimes)
• Microsoft Systems Journal (www.microsoft.com/msj)
• Seans Web Page (www.eburg.com/baxters/)

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May the COM Be With You

• Thank You