You may receive an LNK2019 error message when you build a Visual C++ 2005 or Visual C++ .NET application that uses a CString-derived class from a DLL file

Steps to Reproduce the Problem

1. In Visual C++ 2005 or in Visual C++ .NET, create a default MFC DLL project.
2. On the Project menu, click Add New Item.
3. Name the file MyString.h. Select the Header File template, and then click Open.
   
   Note In Visual Studio 2005, click Add instead of Open.
4. Add the following code to MyString.h:

   #ifdef _USRDLL
   #define IMPEXP __declspec(dllexport)
   #else
   #define IMPEXP __declspec(dllimport)
   #endif 
   
   class IMPEXP CMyString: public CString
   {
      public:
         CMyString();
         virtual ~CMyString();
   
         void MyMethod();
   };
   					

5. Add a new C++ source file to the project named MyString.cpp.
6. Add the following code to MyString.cpp:

   #include "StdAfx.h"
   #include "mystring.h"
   
   CMyString::CMyString(void) {}
   CMyString::~CMyString(void) {}
   
   void CMyString::MyMethod(void)
   {
      AfxMessageBox( T ("foo is called") );
   
   }
   					

7. Save and build the DLL project.
8. Create a new Win32 Project. In the wizard, change the setting under Application Settings to the Console Application type, and then click to select the MFC checkbox to add MFC support.
9. In the new application's main source file, include MyString.h from the DLL project. Verify that the correct full path or relative path is being used.
10. Before the main function, add a #pragma to link to the library for the DLL (again, be sure to use a correct full path or relative path), as follows:

    #pragma comment(lib, "..\\Debug\\TestDll.lib")
    					

11. Add the following code in the else block of the main function:

    // TODO: code your application's behavior here.
    CString strHello = T ("Hello") ;
    CMyString* myStr = new CMyString();
    myStr->MyMethod(); 
    					

12. Build the project.
    
    The build fails with the "LNK2019" error.
13. In the console application, add a new C++ source file named Simple.cpp.
14. Add the following code to Simple.cpp:

    #include "stdafx.h"
    CString bar()
    {
       CString s =<?xm-insertion_mark_start author="v-bobbid" time="20080319T140037-0800"?> T (<?xm-insertion_mark_end?><?xm-deletion_mark author="v-bobbid" time="20080319T140041-0800" data=" "?>"Hi"<?xm-insertion_mark_start author="v-bobbid" time="20080319T140045-0800"?>)<?xm-insertion_mark_end?>;
       return s;
    }
    					

15. Build the project again.
    
    You receive the "LNK2005" and "LNK1169" error messages.
16. Apply the solution provided in the "Resolution" section of this article.
    
    The project should now build without errors.

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