COM using C for Beginners Ranjit R. Sawant Developer Support Engineer Office Integration Microsoft C

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COM using C for BeginnersRanjit R.
SawantDeveloper Support EngineerOffice
Integration Microsoft Corporation
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Agenda

• Component architecture
• Component Object ModelCOM
• Interfaces
• IUnknown
• Component life time
• Components and servers
• HRESULTS and GUIDS

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Agenda (continued)

• COM registry entries
• COM library functions
• Class factories
• Server activation and component creation
• COM servers in EXE
• Interface description language (IDL)
• MIDL
• Type libraries

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Component Architecture
Component Application
Monolithic Application
Component C
Component A
Component D
Component B
Component Application
Component C
Component A
New Improved ,Component D
Component B
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Component Architecture (continued)

• Component benefits
• Application evolves over time
• Application customization
• Component libraries
• Distributed components
• Component Requirements
• Dynamic linking
• Encapsulation

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Component Architecture (continued)

• Encapsulation leads to following constraints
• Components should have language independence
• Components should be in binary form
• Components must be upgradeable
• Components must be transparently relocateable on
  the network

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Component Object Model

• Component object model (COM) is a specification
  for a way of creating components and building
  applications from these components.
• COM provides a library of standard functions.

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Component Object Model (continued)

• COM components meet all the requirements of
  component architecture
• COM components link dynamically.
• COM components can be encapsulated
• Language independent
• Binary form
• Upgradeable
• Relocateable

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Interfaces

• An interface provides a connection between two
  different objects.
• Implementing a COM interface in Visual C

// define interface struct interface IX
virtual void __stdcall Fx1()0 virtual void
__stdcall Fx2()0
Class IX public virtual void Fx1() 0
virtual void Fx2() 0
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Interfaces (continued)

• Interface Features
• Interfaces are immutable
• Interfaces provide polymorphism
• Memory layout of an interface

interface IX
IX
pIX virtual
void __stdcall Fx1() 0 virtual void
__stdcall Fx2() 0 virtual void __stdcall
Fx3() 0 virtual void __stdcall Fx4() 0

Virtual
Function Table
vtbl pointer
Fx1
Fx2
Fx3
Fx4
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Interfaces (continued)

• Instance Data for the class and Vtbl

Client IX
Class pIX
Virtual Function
Table
vtbl pointer
Fx1
Fx1 Fx2 Fx3 Fx4
Fx2
m_data1
Fx3
m_data2
Fx4
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IUnknown

• Definition of IUnknown

interface IUnknown virtual HRESULT __stdcall
QueryInterface(const IID iid, void ppv) 0
virtual ULONG __stdcall AddRef() 0 virtual
ULONG __stdcall Release() 0
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IUnknown (continued)

• Every COM Interface derives from Iunknown.

interface IXIUnknown virtual HRESULT
__stdcall Fx() 0 Memory layout of IX
Client IX
Class pIX
Virtual Function
Table
vtbl pointer
QueryInterface
QueryInterface AddRef Release
Fx
AddRef
Release
Fx
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QueryInterface

• QueryInterface definition
• Client using QueryInterface

HRESULT __stdcall QueryInterface(const IID iid ,
void ppv)
void CallFx(IUnknown pI) //Define
a pointer to the interface that you need
IX pIX NULL   HRESULT hr
pI-gtQueryInterface(IID_IX, (void)pIX)
if (SUCCEEDED(hr)) //Check the return
value pIX-gtFx()
//Use Interface to call method Fx on it
 
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QueryInterface (continued)

• Component implementing QueryInterface

Class CApublic IX , public IY. //class
implements IX and IY interfaces   HRESULT
__stdcall CAQueryInterface(const IID iid,
void ppv) if (iid IID_IUnknown)
ppv static_castltIXgt(this) //The
client wants the IUnknown interface
else if (iid IID_IX) ppv
static_cast ltIXgt(this) //The client wants the
IX interface else if (iid IID_IY)
ppv static_cast ltIYgt(this) //The
client wants the IY interface else
ppv NULL return
E_NOINTERFACE static_castltIUnknowngt(p
pv)-gtAddRef() //for reference counting
return S_OK  
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QueryInterface (continued)

• QueryInterface Rules
• You always get the same IUnknown
• You can get an Interface if You Got it Before
• QueryInterface is Symmetric
• QueryInterface is Transitive
• QueryInterface is Reflexive

QI(IX)-gtIUnknown QI(IY)-gtUnknown
If QI(IX)-gtIY is true once, then QI(IX)-gtIY is
true always
If QI(IX)-gtIY is true, then QI(QI(IX)-gtIY)-gtIX
must be true as well
If QI(QI(IX)-gtIY)-gtIZ is True ,Then QI(IX)-gtIZ
must be true as well
QI(IX)-gtIX
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Component Life Time

• In COM, the component should take care of
  controlling its own life time
• Reference counting
• Implementing AddRef and Release

ULONG __stdcall AddRef() return m_cref
//m_cref is a private member of the
class. ULONG __stdcall Release() if (
--m_cRef 0) delete this
return 0 return m_cRef  
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Rules of Reference Counting

• Call AddRef before returning
• Call Release when you are done
• Call AddRef after assignment

HRESULT __stdcall CAQueryInterface(const IID
iid, void ppv) if (iid IID_IUnknown)
ppv static_castltIXgt(this) //The
client wants the IUnknown interface .
static_castltIUnknowngt(ppv)-gtAddRef()
return S_OK
HRESULT hr pIUnknown-gtQueryInterface(IID_IX,(voi
d)pIX) if (SUCCEEDED(hr)) pIX-gtFx()
//Use interface IX. pIX-gtRelease() //Done
with IX. Call Release on it.
IX pIX2 pIX //Make a copy of
pIX. pIX2-gtAddRef() //Increment the reference
count
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Components and Servers

• COM component is a unit of functionality that
  implements one or more interfaces.
• COM object server is either an executable or a
  DLL that hosts and can create one or more COM
  components.

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COM Object Server in a DLL

• COM DLL server exports the following five
  functions
• DllMain
• DllCanUnloadNow
• DllGetClassObject
• DllRegisterServer
• DllUnregisterServer

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HRESULTS and GUIDS

• HRESULT
• 31 30
  16 15
  0
• Globally Unique Identifier or GUID

FACILITY
RETURN CODE
typedef struct _GUID DWORD Data1
WORD Data2 WORD Data3 BYTE
Data48 GUID   Examples of GUID
16622323-AE18-11CF-A6AE-0080C7B27789 32bb8129,
0xb41A, 0x11CA,0xA6,0Xbb,0x0,0x80,

0xC7,0xB2,0xD6,0x82  
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COM Registry Entries

• COM components use registry to convey information
  about the location of their servers.
• A COM DLL server is required to make these
  entries when the exported function
  DLLRegisterServer is called.
• The COM library function CoGetClassObject takes
  in a Class ID of the component and then looks
  into the registry to find the file name of the
  server in which the component is hosted.

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COM Registry Entries (continued)
My Computer

• HKEY_CLASSES_ROOT

CLSID
16685BA0-8890-2343-A873-00340ACE09AC
My Adder Component
InprocServer32
C\Adder\MyAddercomponent.dll
ProgID
Adder.MyAdderComponent.1
Adder.MyAdderComponent.1
My Adder Component
CLSID
16685BA0-8890-2343-A873-00340ACE09AC
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COM Registry Entries (continued)

• Other COM registry entries
• PROGID or Programmatic ID
• AppID
• Component Categories
• Interface
• TypeLib

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COM Library Functions

• The COM Library is implemented in Ole32.dll.
• Some important COM Library functions
• CoInitializeEx
• CoUninitialize
• CoGetClassObject
• CoCreateInstance
• This is a helper function implemented as
  follows

CoGetClassObject(rclsid,dwClsContext,NULL,
IID_IClassFactory, pCF) hresult
pCF-gtCreateInstance(pUnkOuter,riid,ppvObj)
pCF-gtRelease()
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Class Factories

• A class factory is a COM component that creates
  other components that correspond only to a
  single, specific CLSID.
• The standard interface for creating components is
  IClassFactory or IClassFactory2.
• Declaration of IClassFactory

interface IClassFactoryIUnknown HRESULT
__stdcall CreateInstance(IUnknown pUnknownOuter,
const IID iid,
void ppv)
HRESULT __stdcall LockServer(BOOL bLock)  
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Server Activation and Component Creation

• Client creating a component

COM LIBRARY
COM DLL
CLIENT
1Calls CoGetClassObject
2 Calls DLLGetClassObject

Calls CoGetClassObject pIClassFactory pIX


CoGetClassObject
DllGetClassObject
3 Creates Class Factory
4 Returns IClassFactory
5 Calls IClassFactoryCreateInstance
IClassFactory
7Returns IX
6Creates Component
8Calls IXFx
IX
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Implementation of DLLGetClassObject
STDAPI DLLGetClassObject(const CLSID clsid,
const IID iid, void ppv) CFactory1
pFactory1 if (clsid CLSID_Comp1) // Is
the requested Component hosted by us? pFactory1
new Cfactory1 //create Class Factory
associated with CLSID_Comp1 if (pFactory1
NULL) return E_OUTOFMEMORY
HRESULT hr pFactory1-gtQueryInterface(iid,ppv)
//Get the requested interface
pFactory1-gtRelease() return hr else if
(clsid CLSID_Comp2) //create
Class Factory associated with CLSID_Comp2 else
//return error return CLASS_E_CLASSNOTAVAILABLE

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Implementation for IClassFactoryCreateInstance
HRESULT __stdcall CFactory1CreateInstance(IUnkno
wn pUnknownOuter,

const IID iid, void ppv)
//Create component CA pA new CA if (pA
NULL) return E_OUTOFMEMORY //get the
requested interface HRESULT hr
pA-gtQueryInterface(iid,ppv)   //Release the
IUnknown pointer pA-gtRelease() return hr
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COM Servers in EXE

• Local Procedure call, marshalling, and Proxy/stub
  DLLs
• Function call for out-of-proc components

EXE
EXE
CLIENT
COM COMPONENT
PROCESS BOUNDARY
STUB unmarshals parameters.
PROXY marshals parameters.
Local Procedure Call
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Interface Description Language

• Interface Description Language (IDL) is used to
  describe COM interfaces in a standard way.
• IDL is used to create proxy/stub DLLs and type
  library.

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Sample IDL File
//IX.IDL //Import other IDL definitions to be
used in this file import
unknwn.idl , iy.idl , iz.idl //Interface
IX //interface
header object , //defines a COM
interface uuid(32455554-ad34-1213-a55e-0023bcd3456
a), //IID helpstring(IX Interface), //help
string that appears in TypeLibrary pointer_default
(unique) //how to treat pointers if no attributes
is given interface IX IUnknown
//interface body HRESULT FxStringIn(in,string
wchar_t szIn) //in describes input
HRESULT FxStringOut(out,string wchar_t
szout) //out describes output HRESULT
GetIY(out IY pIMy) //IY described in iy.idl
HRESULT GetIZ(out IZ pIZ) //IX described
in iz.idl   //describe other interfaces .
       
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MIDL

• Microsoft IDL (MIDL) compiler is used to generate
  proxy and stub DLLs from IDL. MIDL also generates
  other helper files.
• Command line midl IX.idl
• Files generated by MIDL compiler

IX.H
IX_P.C
C Compiler and Linker
MIDL.EXE
IX.IDL
IX.DLL
IX_I.C
DLLDATA.C
makefile
IX.DEF
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Type Libraries

• Type library provides type information of
  components, interfaces, methods, properties,
  arguments, and structures.

 //sample.idl import unknwn.idl // Define
Interface IX . //Component and Type Library
description uuid(D3011AE1-B943-9800-A558-00238709
1234),version(1.0),helpstring(Type
library) library SampleLib //Component
class description uuid(0CA06289-AA10-9800-A558-
002387091234), helpstring(MyComponent
description) coclass MyComponent
default interface IX
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Type Libraries (continued)

• Distributing type libraries
• Using type libraries
• LoadRegTypeLib
• LoadTypeLib
• RegisterTypeLib
• UnRegisterTypeLib
• Importing type library definitions in C
• import "filename" attributes

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