1. Microsoft’s Distributed Component Object Model (DCOM)
A semi-technical overview
Jim Ries
Updated 10/5/1999

2. Genealogy DCOM comes from COM and OSF (now Open Group) DCE
Remote Procedure Calls (RPC)
Interface Definition Language (IDL)
Component Object Model (COM)
ORPC
OMG CORBA - a parallel standard
Different RPC
Different IDL
COM proxy == CORBA stub
COM stub == CORBA skeleton

3. Microsoft proprietary, but . . .
Open Group’s COMSource:
Software AG’s EntireX:
The Active Group:

4. COM Goals Encapsulation (separate implementation from interface)
Versioning
Execution context independence
Language independence
Object Creation / Lifetime Control
Standard error code (HRESULT)
Solve object discovery problem
Scripting
The Holy Grail of Reuse

5. Alphabet soup: COM/OLE/ActiveX
COM is a binary standard and a style for creating objects.
OLE is (was) a set of COM interfaces for embedding documents (originally “Object Linking and Embedding”).
ActiveX is a marketing buzz-word meaning COM and/or OLE, but usually applied to Internet-oriented components.

6. Later and later binding
“Editor inheritance” binds at compile time.
Link libraries (.LIB) bind to “components” at link time.
Dynamic link libraries (.DLL) bind at run time, but need a header at compile time, and path at runtime.
COM components bind at runtime and may need neither a header, nor a path! (though typelib contains header-like “meta-data”)

7. Interfaces
COM enforces the concept of interfaces being separate from implementation.
Interface == Abstract Base Class
Objects support multiple interfaces through multiple inheritance.
Interfaces NEVER change!
A “control” is just a COM component with the right interfaces.

8. GUID’s (or UUID’s)
Globally Unique Identifiers (Universally Unique Identifiers)
Needed to avoid name collisions
A class is associated with a GUID (CLSID).
An interface is associated with a GUID (IID).
The Windows Registry: a hierarchical database.

9. Execution Context In proc - DLL’s (no marshalling)
Out of proc - EXE’s (LRPC)
Remote - EXE’s using DCOM
RPC
DCE “compatible” (see “Interconnecting Personal Computers with the Distributed Computing Environment” by Jim Ries, UMC Thesis, 1998.)

10. Coding Tools C - Raw C++ - Raw C++ - ATL C++ - MFC Visual Basic
J++ (pseudo Java)
Binary standard ==> any language COULD produce COM components.

11. Platforms Win32 Unix platforms (with some help)
Windows 95 (DCOM as separate download; included in OSR2)
Windows NT 4.0
Windows 98
Windows 2000
Unix platforms (with some help)

12. Nuts and Bolts CoInitialize() CoCreateInstance() IUnknown
QueryInterface()
AddRef()
Release()
CoUninitialize()

13. Demonstration - IDL [ object,
uuid(75D873CD-7B63-11D3-9D43-00C0F031CDDE),
helpstring("IServer Interface"),
pointer_default(unique) ]
interface IServer : IUnknown {
HRESULT Hello([in, string] char * pszMessage); };

14. Demonstration - Server Code
// Prototype
class CServer :
public IServer, public CComObjectRoot, public CComCoClass<CServer,&CLSID_Server> {
// Some code omitted for brevity
// IServer
public:
HRESULT STDMETHODCALLTYPE Hello(unsigned char * pszMessage); };
// Code
HRESULT STDMETHODCALLTYPE CServer::Hello(unsigned char * pszMessage)
char szBuf[256];
wsprintf(szBuf,"%s",pszMessage);
::MessageBox(0,szBuf,"Server",MB_OK);
return(S_OK); }

15. Demonstration - Client Code
if (SUCCEEDED(
hr=CoCreateInstance(CLSID_Server,NULL,
CLSCTX_LOCAL_SERVER,
IID_IServer,(void **)&pServer))) {
if (SUCCEEDED(hr=pServer->Hello((unsigned char *)"Hello from the client")))
MessageBox("Client: Server printed the message");
else
wsprintf(szBuffer,"Hello() method failed: 0x%lX.\n",hr);
MessageBox(szBuffer); }
pServer->Release();
wsprintf(szBuffer,"Unable to create a server: 0x%lX.\n",hr);

16. Distributed Scenario
From “DCOM Architecture” a Microsoft white paper.

17. Demonstration
Run DCOM “Hello world” demo here.

18. Additional Technologies
COM+
MTS - Microsoft Transaction Server
MSMQ - Microsoft Message Queue
Compiler supported IUnknown, etc.
ADS - Active Directory Service
As “distributed registry”
As namespace abstraction
All Microsoft products are COM based:
IIS - Internet Information Server
Exchange
Internet Explorer
Word, Excel, etc.

19. References Microsoft DCOM page IETF DCOM Standard Proposal
Inside OLE by Kraig Brockschmidt, Microsoft Press, 1995.
Essential COM by Don Box, Addison Wesley, 1998.
Inside COM by Dale Rogerson, Microsoft Press, 1997.
Don Box homepage
ActiveX COM Control Programming by Sing Li and Panos Economopoulos, Wrox Press, 1997.
COM-CORBA Interoperability by Geraghty, et. al., Prentice Hall, 1999.
Microsoft Developer Network