COM Component Object Model

1
COM (Component Object Model)
2
COM

• COM Server
• DLL (in-process) or EXE (out-of-process
  marshalling)
• Register CLSID (GUID) in windows register
• Interface class
• Consists of pure virtual functions
• Implementation of Interface class
• Inherit from the Interface
• Implement the pure virtual functions
• COM Client
• Find the DLL in windows register
• Find a interface
• Call functions of the interface
• COM Library
• O/S services supporting COM

3
COM Support in Visual C

• MFC Library
• ATL (ActiveX Template Library)
• Provided with Source Code
• Use template Class
• Small Size

4
COM Interface

• IUnknown
• Root interface
• Methods
• Addref ()
• Increase count
• Release ()
• QueryInterface (IID)

5
COM Server Implementation

• Project MFC DLL
• CLSID and IID generation
• Viusalstudio\common\tools\guidgen.exe
• Registration of CLSID
• HKEY_CLASSES_ROOT\CLSID directory
• RegSvr32.exe
• Call DllRegisterServer() function of DLL
• Interface implementation
• ?) Add(), Sub()
• CoClass imlementaion
• Addref()
• Release()
• QueryInterface()
• Add(), Sub()
• Class Factory Implementation
• O/S calling function implementation and DEF file
  editing
• DllUnRegisterServer()
• DllRegisterServer()
• DLLGetClassObject()

6
CoClass (Component Object Class)

• Consists
• interfaces
• its implementation
• CLSID
• COM Object
• Class Factory
• CoClass
• Generats CoClasses in a DLL
• Interface IClassFactory
• Methods CreateInstance, LockServer

7
COM Applications

• Automation
• Script language Com client
• IDispatch interface
• OLE (Object Linking and Embedding)
• Example Excel data in MS Word
• Interface IOleContainer, IOleObject, etc
• Clipboard
• Interface IDataObject
• DragDrop
• Interface IDropTarget, IDropSource
• ActiveX Control (.ocx)

8
COM Client

• GoGetClassObject()
• Generate COM object
• QueryInterface()
• Get the pointer of interface
• Call the methods of the interface
• Release()
• Free COM object from memory