How to Support Two File Extensions per MFC Document Type

WINDOWS             | WINDOWS NT

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with:

      - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
      - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0
  

SUMMARY

Applications built with the Microsoft Foundation Classes (MFC) document or view architecture have at most one file extension associated with each document type. This default file extension, if specified, is stored in the document template string stored in the string table.

It is often useful to associate two file extensions with a given document type. This article describes a technique you can use to allow two file extensions to be stored in the document template string. Through class derivation and function overrides, it is possible to associate both file extensions with the document template.

MORE INFORMATION

Step-by-Step Procedure

Use the following steps to associate two file extensions with a single document type in either SDI or MDI applications.

1. Modify the string table entry so that it contains two file extensions. The two extensions are entered into the filterExt field separated by a semicolon (for example, .aaa;.bbb). The document template string may look similar to this:

   \nExts\nExts\nFiles (*.aaa; *.bbb)\n.aaa;.bbb\nExts.Doc\nExts Doc.

2. Derive a class from CMultiDocTemplate for MDI applications, or CSingleDocTemplate for SDI applications. Add this class to your project, and use it when creating the document templates in your InitInstance function. You will need to create a constructor that simply calls the base class constructor.

   CMyMultiDocTemplate::CMyMultiDocTemplate(

         UINT nIDResource, CRuntimeClass* pDocClass,
         CRuntimeClass* pFrameClass, CRuntimeClass* pViewClass ) :
         CMultiDocTemplate(nIDResource, pDocClass, pFrameClass, pViewClass)
         { };
   

3. Override the GetDocString function in your class derived from CMultiDocTemplate or CSingleDocTemplate in step 2.

   BOOL CMyMultiDocTemplate::GetDocString(CString& rString,

                               enum DocStringIndex i) const
      {
           CString strTemp,strLeft,strRight;
           int nFindPos;
           AfxExtractSubString(strTemp, m_strDocStrings, (int)i);
   
           if(i == CDocTemplate::filterExt)  {
             nFindPos=strTemp.Find(';');
             if(-1 != nFindPos) {
               //string contains two extensions
               strLeft=strTemp.Left(nFindPos+1);
               strRight="*"+strTemp.Right(lstrlen((const
               char*)strTemp)-nFindPos-1);
               strTemp=strLeft+strRight;
             }
           }
       rString = strTemp;
       return TRUE;
      }
   
   

4. Override CMyMultiDocTemplate::MatchDocType so that both file extensions are recognized when a file is opened.

   CDocTemplate::Confidence CMyMultiDocTemplate::MatchDocType(const

           char* pszPathName, CDocument*& rpDocMatch)
      {
           ASSERT(pszPathName != NULL);
           rpDocMatch = NULL;
   
           // go through all documents
           POSITION pos = GetFirstDocPosition();
           while (pos != NULL)
           {
              CDocument* pDoc = GetNextDoc(pos);
              if (pDoc->GetPathName() == pszPathName) {
                 // already open
                 rpDocMatch = pDoc;
                 return yesAlreadyOpen;
              }
           }  // end while
   
           // see if it matches either suffix
           CString strFilterExt;
           if (GetDocString(strFilterExt, CDocTemplate::filterExt) &&
             !strFilterExt.IsEmpty())
           {
              // see if extension matches
              ASSERT(strFilterExt[0] == '.');
              CString ext1,ext2;
              int nDot = CString(pszPathName).ReverseFind('.');
              const char* pszDot = nDot < 0 ? NULL : pszPathName + nDot;
   
              int nSemi = strFilterExt.Find(';');
              if(-1 != nSemi)   {
                // string contains two extensions
                ext1=strFilterExt.Left(nSemi);
                ext2=strFilterExt.Mid(nSemi+2);
                // check for a match against either extension
                if (nDot >= 0 && (lstrcmpi(pszPathName+nDot, ext1) == 0
                    || lstrcmpi(pszPathName+nDot,ext2) ==0))
                  return yesAttemptNative; // extension matches
              }
              else
              { // string contains a single extension
                if (nDot >= 0 && (lstrcmpi(pszPathName+nDot,
                    strFilterExt)==0))
                return yesAttemptNative;  // extension matches
              }
           }
           return yesAttemptForeign; //unknown document type
      }
   
   

5. To make Save and Save As function correctly, override the DoSave function in your CDocument-derived class. Simply cut and paste the MFC implementation of CDocument::DoSave() from Mfc\Src\Doccore.cpp into your derived class.

   Replace these lines:

           // append the default suffix if there is one
           CString strExt;
           if (pTemplate->GetDocString(strExt, CDocTemplate::filterExt) &&
               !strExt.IsEmpty())
           {
              ASSERT(strExt[0] == '.');
              newName += strExt;
           }
   
      with these lines:
   
           // append the default suffix if there is one
           CString strExt;
           if (pTemplate->GetDocString(strExt, CDocTemplate::filterExt) &&
               !strExt.IsEmpty())
           {
             ASSERT(strExt[0] == '.');
   
             int nSemi;                       //added
             if(nSemi = strExt.Find(';'));    //added
             strExt = strExt.Left(nSemi);     //added
   
             newName += strExt;
           }
   
      The three added lines of code return the first extension by default when
      Save As is processed for a document type that has two extensions
      specified in its document string. The user must type over this extension
      if a file is to be saved with the second extension specified in the
      document string.
   
      You will also need to replace the following lines of code:
   
         CATCH_ALL(e)
         {
         TRACE0("Warning: failed to delete file after failed SaveAs.\n");
         DELETE_EXCEPTION(e);
         }
   
      with:
   
         CATCH_ALL(e)
         {
         TRACE0("Warning: failed to delete file after failed SaveAs.\n");
         e->Delete();     //modified
         }
   
      This is necessary because the DELETE_EXCEPTION() macro is undefined in
      this context.
   
   

6. (16-bit Editions Only) Use Class Wizard to provide OnFileSave and OnFileSaveAs command handlers in your derived document class. This is necessary because in the 16-bit versions of Visual C++, the DoSave function is not virtual. Simply cut and paste the contents of the base class versions of both functions from Mfc\Src\Doccore.cpp into your command handlers. The resulting functions call the local copy of DoSave rather than the base class version. It may be necessary to add #include "io.h" to your project to provide a definition for the _access function.