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Relationship between APPID & CLSID?

I have created a dll using VB5 and registered it on an NT4 server using Microsoft Transaction Server. I can use DCOMCNFG to view the appid of the VB application on the client machine and I can view the properties of the component in MTS to see the cslid on the server.
I have been asked to explain how the two are related and I have no idea. I believe that the clsid is somehow attached to the appid to form a logical link so that the client side application knows where to look for the server side dll. Am I close? Can you offer another explanation? Thanks.
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rodnolan
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rodnolan
1 Solution
 
Tommy HuiEngineerCommented:
A CLSID is the ultimate identifier for any creatable objects. For these creatable objects, the CLSIDs are stored in the registry under HK_CLASS_ROOT\CLSID. Normally, these objects are provided by a local server (either EXE or DLL). However, for distributed components there may be more than one object provided by the component and hence these are grouped into an APPID that identifies the component. These APPIDs are stored in HK_CLASS_ROOT\APPID. But for every object that is a part of that APPID, under HK_CLASS_ROOT\CLSID\{GUIDofCreatableObject}\AppID = {GUIDofAPP}.

So when you create the object, COM looks into the APPID to see where the object is located. If there is an APPID associated with the CLSID, it uses DCOM to activate the server. If there isn't, then it checks the usual places (LocalServer32, InProcServer32, and InProcHandler32) to find it locally.
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rodnolanAuthor Commented:
Thank you. That was a great explanation.
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