Creating DLLs in Windows

I've always seen and hated those weird macros that preceded functions of header files. Why couldn't they be clean and simple? I hated them because I didn't know much about them, they seemed arbitrary, prone to error, non-standard, no way of knowing the real motivation behind. 

Not anymore. Today I had to create an intermediary DLL between two different object file formats and I had to go down to the level of dirty and hacky in the quest to make it work. And I stumbled upon this awesome page that has the "ultimate header file" template. I discovered half the things there myself, but it nailed the coffin of problems for me. 

Here are the problems that I had to tackle in general:

•  You can't have both __declspec(dllexport) and __declspec(dllimport) in your header, that's why you can't use the same header for both compiling the library and the user of the library.
• When the user of the library prefers C and your library is in C++, you are hosed because your library has a symbol like (__imp_?calculateSquare@@YAHH@Z) and the user's object file has __imp__calculateSquare. Therefore you get linker errors. C and C++ don't mix.

#ifndef _DEFINED_uniqueheadername  //---------------1---------------+
#define _DEFINED_uniqueheadername  //---------------1---------------+ 
                                                                 // | 
  #if _MSC_VER > 1000  //--------------2---------------+            |
    #pragma once                                    // |            |
  #endif //----------------------------2---------------+            |
                                                                 // |
  #ifdef __cplusplus  //---------------3---------------+            |
  extern "C" {                                      // |            |
  #endif // __cplusplus  //------------3---------------+            |
                                                                 // |
  #ifdef _COMPILING_uniqueheadername  //-----------4-----------+    |
    #define LIBSPEC __declspec(dllexport)                   // |    |
  #else                                                     // |    |
    #define LIBSPEC __declspec(dllimport)                   // |    |
  #endif // _COMPILING_uniqueheadername  //--------4-----------+    |
                                                            // |    |
  LIBSPEC linkagetype resulttype name(parameters);          // |    |
  // ... more declarations as needed                        // |    |
  #undef LIBSPEC   //------------------------------4-----------+    |
                                                                 // |
  #ifdef __cplusplus    //----------------5---------------+         |
  }                                                    // |         |
  #endif // __cplusplus  //---------------5---------------+         |
#endif // _DEFINED_uniqueheadername //-----------------1------------+