当前位置: 主页 > 浏览 >

献给广大需要在VC6中用MFC打开Access 2000数据库的朋友(Microsoft的解答)

收藏 时间:2009-12-27 来源:查看 收藏:yuxinyue1987 阅读:384 标签:解答  朋友  数据库  打开  需要  广大  
Q I need to work with a Microsoft Access 2000 database and Data Access Objects (DAO) 3.6 from C++ with MFC. When I try to use CDaoRecordset, I get an "Unrecognized database format" error message. MFC probably uses DAO 3.5. How can I open files in MFC

Q I need to work with a Microsoft Access 2000 database and Data Access Objects (DAO) 3.6 from C++ with MFC. When I try to use CDaoRecordset, I get an "Unrecognized database format" error message. MFC probably uses DAO 3.5. How can I open files in MFC that were created with Microsoft Access 2000?

Ing. Jozef Sakalos
Slovakia

A This question is also discussed in the Knowledge Base article Q236991 (http://support.microsoft.com/support/kb/articles/ Q236/9/91.asp), but since so many people have asked, I'll cover it here. The problem is that MFC is using the wrong DAO DLL. When you call AfxDaoInit to initialize DAO for MFC, MFC first determines which version of the DAO engine to load.


 // in daocore.cpp
 void AFXAPI AfxDaoInit()
 {
 •
 •
 •
   BYTE bUseDao = _AfxDetermineDaoVersion();
   switch (bUseDao) {
   case 35:
     // Use DAO350.DLL
     break;
   case 30:
     // Use DAO300.DLL
     break;
   case 36:
     // Use DAO360.DLL
     break;
   }
 }
_AfxDetermineDaoVersion is a macro that expands to 30, 35, or 36, depending on which version of MFC you have.

 #if _MFC_VER >= 0x0601
 #define _AfxDetermineDaoVersion() (36)
 #else
 #define _AfxDetermineDaoVersion() (35)
 #endif
      The previous code only shows how _AfxDetermineDaoVersion expands when you link with a static MFC (_AFXDLL not #defined). If you're linking with MFC as a DLL (_AFXDLL #defined), _AfxDetermineDaoVersion expands to a real inline function:

 static inline BYTE _AfxDetermineDaoVersion() 
 {
   BYTE bReturn = 35;
   AFX_MODULE_STATE* pModuleState = AfxGetModuleState();
   if (pModuleState->m_dwVersion < 0x421)
     bReturn = 30;
   else if (pModuleState->m_dwVersion >= 0x0601)
     bReturn = 36;
   return bReturn;
 }
      The only difference between these cases is that in the first situation, the MFC version number is hardcoded as _MFC_ VER. For dynamically linked projects, MFC gets the version from your module's state. This is required because apps with different MFC versions can potentially call the same MFC42.DLL. Either way, MFC uses its own version number to determine which version of DAO to use. If the MFC version is 0x0601 (6.1) or higher, MFC uses DAO 3.6, the version used by Microsoft Access 2000.
      So in order to gain access to Microsoft Access 2000 databases, you need MFC version 6.1. There's only one problem: there is no version 6.1 of MFC. The current version is 6.0!

 // in afxver_.h
 #define _MFC_VER 0x0600
      The only way I know to get MFC 6.1 requires a time-travel device. So the question is: what's going on here? If there's no version 6.1, why does daocore.cpp have 0x0601 in it? The obvious way to support a different DAO version would have been to put the DAO version number as an argument to AfxDaoInit—after all, why should the DAO version be tied to the MFC version? But who thinks of version numbers the first time they write the code? But once a signature for AfxDaoInit was chosen, the MFC folks were forever constrained because they had to maintain strict binary backward compatibility with MFC42.DLL. Adding an argument to AfxDaoInit would change the function signature and hence the DLL entries. So the problem confronting the Redmondtonians was: how to tell MFC to use the new DAO, without introducing any new functions or function arguments? Using the MFC version number provided a convenient, if kludgy, way. (Of course, I'm only guessing that's what happened.)
      So, to use the latest DAO engine, all you have to do is make MFC pretend it's the mysterious nonexistent version 6.1. If you're building with MFC as an MFC DLL, all you have to do is write:

 AfxGetModuleState()->m_dwVersion = 0x0601; 
 AfxDaoInit();
That is, set the version number in your module state to 6.1 before you call AfxDaoInit. If you're the paranoid sort—and who isn't paranoid from time to time—you can save the original version and restore it after doing your business. A simple grep reveals, however, that _AfxDetermineDaoVersion is the only function in all of MFC that uses the mysterious 0x0601 version, so changing the version to 0x0601 should have no side effects.
      If you're using the static MFC library, life is a little more difficult. The MSDN article mentioned earlier advises that you edit daocore.cpp and rebuild MFC. Yikes! That's a bit drastic for my taste. You should never rebuild MFC if you can avoid it. Better to confine whatever workarounds you need to your own project. Life is safer and more predictable if you make a hard and fast rule that the MFC libraries and code hierarchy are off limits. In fact, you should do what I do as soon as I install any new version of MFC or any SDK—set the read-only attribute on all the files.
      But then how can you make MFC use DAO360.DLL in a static MFC build? Easy. Just copy the offending MFC source file (in this case, daocore.cpp) to your code directory, make the changes, and add it to your project. This is always a better way to modify MFC than munging MFC directly. When the linker needs a function or symbol from daocore.obj, it'll find the one in your modified version and, therefore, never need to load daocore.obj from MFC42.LIB. Of course, most MFC modules won't compile outside of their context; there are always some #include files from \mfc\src that you need. The easiest way to find them is just keep compiling and fixing until there are no more errors to fix. For daocore.cpp it turns out you need afximpl.h and daoimpl.h. afximpl.h is the MFC main internal include file, but in fact the only thing daocore.cpp needs from afximpl.h is _countof, so it's easier to copy the definition.

 // Add these lines to the top of daocore.cpp
 #include "daoimpl.h"
 #define _countof(array)    (sizeof(array)/sizeof(array[0]))
 #undef _MFC_VER
 #define _MFC_VER 0x0601
      Once you add these lines to daocore.cpp and copy daoimpl.h to your project, daocore.cpp will compile with only a single warning, which you can safely ignore. And since you've redefined _MFC_VER to 0x0601, _AfxDetermineDaoVersion will now use DAO 3.6 (DAO360.DLL). Problem solved.
      Just don't forget—this is only a stopgap measure! Once the Redmondtonians release the next MFC, you can return to life as usual. But you should remember this general technique: if you're absolutely, positively sure you need to modify MFC, don't modify the source directly. Instead, copy the module you want to change, plus any #include files and #defines it needs, and add it to your project. This method works whether you're linking with MFC as a static or dynamic library.


顶一下
(0)
0%
踩一下
(0)
0%
------分隔线----------------------------
发表评论 所有评论
你还没登录,请先登录后再来评论!
推荐内容
新知先觉