why to put extern C before const IID?

[Lane Yu]

I'm a beginner of COM. I just saw some sample codes: DictionaryObj.cpp extern "C" const IID IID_Dictionary = { 0x54bf6568, 0x1007, 0x11d1, { 0xb0, 0xaa, 0x44, 0x45, 0x53, 0x54, 0x00, 0x00} } ; I don't understand the purpose of extern "C". Since IID_Dictionary will not be exported, why to use this prefix? Thanks.

[Marco M]

The IID has to be visible across all .obj files that are using it. Say IID_IDictionary is defined in fileOne.cpp (which when compiled generates fileOne.obj) And in FileTwo.cpp you are referencing IID_IDictionary, the linker will error out unless the IID has the extern "C", as this extern "C" basically tells the linker that the IID is defined in another obj file. Marco M.