Error link 2001 on CLSID from my another DLL

Question

I have 2 DLLs. I want create instance of class in my second DLL that I declare at .idl file of first library. But I get Error Link 2001 on CLSID, when try to link second DLL. I'm linking with first DLL, so I don't know why I'm getting this error.

Stock.idl

Declaration of class

[uuid(61AF2A3E-8B04-4161-B978-5776BFDF7DB5), helpstring("StockObjectManager Class")]
coclass StockObjectManager
{
    [default] interface IStockObjectManager;
    interface ISQLBase;
};

Generated Files:

Stock.h

EXTERN_C const CLSID CLSID_MazStockObjectManager;

Stock_i.c

MIDL_DEFINE_GUID(CLSID, CLSID_StockObjectManager,0x61AF2A3E,0x8B04,0x4161,0xB9,0x78,0x57,0x76,0xBF,0xDF,0x7D,0xB5);

FIleFromSecondDll.cpp

CComPtr<IStockObjectManager> m_SOManager;
m_SOManager.CoCreateInstance(CLSID_StockObjectManager); // getting Error Link 2001 unresolved external symbol "_CLSID_StockObjectManager"

I want to understand why I'm getting this error and fix it. Of course, I can just #include "Stock_i.c", but it isn't good solution, I guess.

In any linked project (exe or dll) you should have exactly one definition of CLSID_StockObjectManager, so yes you should #include "Stock_i.c", but only in one cpp per project. It's true that c filea should not be #included in general. But COM interface generated c file (..._i.c) with the classid is one exception. — wohlstad
– wohlstad, Commented Jun 20 at 6:48
Do not use generated header file from midl. Make your interfaces oleatomation compatible and use importlib + __uuidof(LibraryNamespace::CoClass), this requires no linking at the client side. — Pepijn Kramer
– Pepijn Kramer, Commented Jun 20 at 6:55
@wohlstad, also I have some DLLs. that's not mine, and there are IDL's generated files and if I link with that extern DLL, I don't need to include there _i.c file. And if I want to use some classes of Windows, I also don't need to include _i.c files. Maybe they somehow export CLSID to .lib file? — SabakaSamson
– SabakaSamson, Commented Jun 20 at 7:14
You will need to link with the _i.c file to make the constant known in your code — Pepijn Kramer
– Pepijn Kramer, Commented Jun 20 at 7:15
@PepijnKramer, I am new to WINAPI, so I have some questions. If I can use uuifof, then whats point of CLSIDFromProgID? Or it is using when I dont have definiton of class that I want to use? And why it's bad idea to use generated header file? — SabakaSamson
– SabakaSamson, Commented Jun 20 at 7:18

RbMm · Accepted Answer · 2025-06-20 09:04:57Z

1

in Stock.h must be not only

EXTERN_C const CLSID CLSID_MazStockObjectManager;

but also

#ifdef __cplusplus
class DECLSPEC_UUID("61AF2A3E-8B04-4161-B978-5776BFDF7DB5") MazStockObjectManager;
#endif

so and use

CComPtr<IStockObjectManager> m_SOManager;
m_SOManager.CoCreateInstance(__uuidof(MazStockObjectManager));

answered Jun 20 at 9:04

RbMm

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