HOWTO: Avoid Error LNK2001 Unresolved External Using DEFINE_GUID

• Microsoft Visual C++ 32-bit Edition, versions 2.0, 2.1, 4.0 4.1, 4.2, 5.0

SUMMARY

A GUID must be initialized exactly once. For this reason, there are two different versions of the DEFINE_GUID macro. One version just declares an external reference to the symbol name. The other version actually initializes the symbol name to the value of the GUID. If you receive an LNK2001 error for the symbol name of the GUID, the GUID was not initialized.

You can make sure your GUID gets initialized in one of two ways:

• If you are using precompiled header files, include the INITGUID.H header file before defining the GUID in the implementation file where it should be initialized. (AppWizard-generated MFC projects use precompiled headers by default.)
• If you are not using precompiled headers, define INITGUID before including OBJBASE.H. (OBJBASE.H is included by OLE2.H.)

MORE INFORMATION

Here is the definition of DEFINE_GUID as it appears in OBJBASE.H:

      #ifndef INITGUID
      #define DEFINE_GUID(name, l, w1, w2, b1, b2, b3, \
                                b4, b5, b6, b7, b8)
          EXTERN_C const GUID FAR name
      #else

      #define DEFINE_GUID(name, l, w1, w2, b1, b2, b3, \
                                b4, b5, b6, b7, b8)
         EXTERN_C const GUID name \
            = { l, w1, w2, { b1, b2,  b3,  b4,  b5,  b6,  b7,  b8 } }
      #endif // INITGUID

In INITGUID.H, you find (among other things):

      #undef DEFINE_GUID

      // Other code . . .

      #define DEFINE_GUID(name, l, w1, w2, b1, b2, b3, \
                                b4, b5, b6, b7, b8)
         EXTERN_C const GUID __based(__segname("_CODE")) name \
                  = { l, w1, w2, { b1, b2,  b3,  b4,  b5,  b6,  b7,  b8 } }

NOTE: It is important to make sure that this is done exactly once for each DLL or EXE. If you try to initialize the GUID in two different implementation files and then link them together, you get this error:

   LNK2005 <symbol> already defined.