今天写了一个DLL，DLL中用到的一个类里用到的MSXML的COM组件，所以在DLL中要包含afx.h头文件，也就不可避免的要用到MFC的类库了，但在编译时出现了错误：mfcs42d.lib(dllmodul.obj) : error LNK2005:_DllMain@12 already defined in NetDriverConfig.obj，
在http://topic.csdn.net/t/20010911/10/281452.html 处的贴子里找到了以下回答，试了一个原来是文件里定义的对象与mfcs42d.lib有冲突，加了命名空间后问题解决。还不知道DLL能不能使用，明天试一下再说了！
Linker Tools Error LNK2005
symbol already defined in object
The given symbol, displayed in its decorated form, was multiply defined.
Tips
One of the following may be a cause:
The most common cause of this error is accidentally linking with both the single-threaded and multithreaded libraries. Ensure that the application project file includes only the appropriate libraries and that any third-party libraries have appropriately created single-threaded or multithreaded versions.
The given symbol was a packaged function (created by compiling with /Gy) and was included in more than one file but was changed between compilations. Recompile all files that include the symbol.
The given symbol was defined differently in two member objects in different libraries, and both member objects were used.
An absolute was defined twice, with a different value in each definition.
Linker Tools Error LNK1169
one or more multiply defined symbols found
The build failed due to multiple definitions of one or more symbols. This error is preceded by error LNK2005.
The /FORCE or /FORCE:MULTIPLE option overrides this error.
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今天又看了下发，发现在WIN32 DLL中使用MFC类库是会存在问题，原因是在MFC/src/DLLModul.cpp 文件中已经声明了DLLMain()函数，所以就不能在项目文件中再声明 DllMain()，否则就会出现如上所示的链接错误。所以在WIN32 DLL中使用MFC类库的话可以不声明DLLMain()函数，如果有操作要放在DLL加载时初始化的话就从CWinApp派生一个类，在项目文件里声明这个对象，在这个CWinApp类的InitInstance()成员函数中做初始化工作