Sharing DLL's with a Managed Extension DLL

The MFC legacy code can use the managed extension classes or the function that uses the managed extension classes by making the derived class a new managed DLL (Dynamic Link Library). Just by incorporating the DLL, managed extension specific library class such as XML related class is freely available to unmanaged applications.

Suppose you have a class called CLabel and want to write its contents in a file in XML format. You make a function such as LoadXMLFile(char* fileName, CLabels* pLabels), which needs to use the XMLTextWriter class. Since XMLTextWriter class must be in a managed extension project, you make it a managed extension DLL and try to call the function -- and then here comes a problem. The main application needs to pass CLabel class to the managed extension DLL.

In other words, to share the data structure from the main application to the managed extension DLL, another DLL must be shared. The figure below shows the DLL's and how they link the other modules. The arrows in the figure indicate the modules link the DLL modules that are pointed at by the arrows.

In the above example, the main application needs CLabel class and it calls

LoadXMLFile(char* fileName, CLabels*
pLabels)

The Main Application

The main application links to both managed extension DLL and the MFC DLL. The application includes the headers for the both modules.


    #include "CLabel.h"
    #include "LabelXML.h"
    ...

    // Using CLabel class
    CLabel* CLabelsDoc::GetLabel(int index)
    {
	    CLabel* pLabel;
	    if(0<=index && index<GetArraySize())
		    pLabel=(CLabel*)m_labels.GetAt(index);
	    else
		    pLabel=new CLabel();
	    return pLabel;
    }

    // Calling LabelXML functions 
    void CLabelsDoc::LoadFile(CString fileName)
    {
	    Initialize();
	    LoadLabelsFile(fileName.GetBuffer(), &m_labels, &m_fileName);
	    LoadImage(m_fileName);
    }
    ...

CLabel Class (MFC DLL)

The CLabel class defines the data structure for the whole application. The class holds the data for a 'label', the caption, the location, etc, and also the methods that are associated with a 'label'.

The header is shared with the DLL and the other modules. Therefore, __declspec(import) or __declspec(import) must be specified. In the header file shown below, the directive is switched with DLLDIR_EXPORT. If the variable is set before including the header file, the export directive will be set. If not, the import directive will be set. The DLLDIR is set to 'undefined' prior to the declaration, since other headers might be using the variable. If you 'undefine' the variable, you can include this for every other DLL headers.


   #undef DLLDIR
   #ifdef DLLDIR_EXPORT
      #define DLLDIR  __declspec(dllexport)
   #else
      #define DLLDIR  __declspec(dllimport)
   #endif

   class DLLDIR CLabel
   {
   public:
	   CLabel(void);
	   ~CLabel(void);

	   CPoint m_point;
	   CStringW m_label;
	   CString m_link;
	   CRect m_box;
	   COLORREF m_color;
	   CPtrArray m_lines;
	   int m_nLines;
	   void MoveLabel(CPoint newLocation);
	   double GetDistance(CPoint point);
	   double GetDistance(CPoint,CPoint,CPoint);
	   CPoint GetClosest(CPoint pos);
	   CPoint GetClosest(CPoint,CRect);
	   BOOLEAN IsOnLine(CPoint,CPoint,CPoint);
	   BOOLEAN IsOnLine(CPoint);
	   void SetLabel(LPWSTR str);	
   };

LabelXML Module (Managed Extension DLL)

LabelXML module is the managed extension DLL. This DLL must import the CLabel DLL module. This DLL in turn to be linked to the main application module which calls the LoadLabelsFile() and SaveLabelsFile(). The header must be shared in the same manner as the CLabel class header.


   #undef DLLDIR
   #ifdef DLLDIR_EXPORT
      #define DLLDIR  __declspec(dllexport)
   #else
      #define DLLDIR  __declspec(dllimport)
   #endif

   extern "C" DLLDIR int LoadLabelsFile(char* fileName, CPtrArray* pArray, CStringW* pImageFileName);
   extern "C" DLLDIR int SaveLabelsFile(char* fileName, CPtrArray* pArray, CStringW* pImageFileName);

Summary

With the managed extension DLL and MFC DLL that defines the data structure, the legacy MFC application can access to the functions that instantiate managed extension classes and successfully uses their methods. This article describes a CLabel example that contains the managed extension DLL and MFC DLL and how the DLL should be linked and specified.