I started building this component by simply creating an ATL COM project, and then adding a simple ATL object. I then added the necessary interfaces and their implementations. A shell extension requires implementation of two interfaces, IShellExtInit and IContextMenu.

IShellExtInit has only one method, Initialize which is used to get the name of the files that have been selected (this extension only gets the name of the file with the current focus, although it could be expanded to be disabled if multiple files were selected, or to put all the paths on the clipboard.)

IContextMenu has three methods to implement, GetCommandString, InvokeCommand, and QueryContextMenu.

• QueryContextMenu is called when the extension is first loaded to get the text and ID for the context menu item.
• InvokeCommand is called when the user invokes the menu item.
• GetCommandString can be called in different situations, but for this example we only care when it is called to get the help text associated with the command.

  I had a bit of trouble getting this to work correctly and I think it has to do with most recent IE installation. Following is the code for GetCommandString:

  
  // IContextMenu::GetCommandString
  HRESULT CCopyPathExt::GetCommandString(UINT /*idCmd*/, UINT uFlags, 
   UINT* /*pwReserved*/, LPSTR pszText, UINT cchMax)
  {
   try {
    USES_CONVERSION;
     // We don't care about the command ID, since we have a 
     // single item
  
     // This is rather funky, but GCS_HELPTEXTA refers to a 
     // single bit (4) and GCS_UNICODE refers to single bit 
     // (1), but GCS_HELPTEXTW is a combination
     // of those values (5).  This works on both NT and 95.
     if ( uFlags & GCS_HELPTEXTA ) {
      // I'm assuming that this value always contains the number 
      // of bytes available for the message, just to be safe.  
      // So half it for Unicode characters.
      const int cChars = cchMax / sizeof(TCHAR);
  
      // Create some storage on the stack for building the 
      // help text
      LPTSTR szHelpText = 
       reinterpret_cast( _alloca( cChars ) );
  
      memset( szHelpText, 0, cChars );
  
      // Create a generic version of the help text depending on 
      // the number of file selected
      if ( 1 == m_cFiles ) {
       _sntprintf( szHelpText, cChars - 1, 
        _T("Copy full path of file to clipboard (%s)"), m_szFile);
      } else {
       const LPTSTR szMultiFileHelpText = 
        _T("Copy full paths of selected files to clipboard");
  
       _tcsncpy( szHelpText, szMultiFileHelpText, min(cChars - 1, 
        _tcslen( szMultiFileHelpText ) ) );
      }
  
      // Do character conversion depending on flag
      if ( uFlags & GCS_UNICODE ) {
       wcsncpy( reinterpret_cast(pszText), 
        T2W(szHelpText), cChars );
      } else {
       strncpy(pszText, T2A(szHelpText), cchMax);
      }
     }
    } catch ( ... ) {
    return E_FAIL;
   }
   return S_OK;
  }
  

Most examples simply test for the bits GCS_HELPTEXTA or GCS_HELPTEXTW, but the value of GCS_HELPTEXTW is actually 5, not a single bit value, which is also equal to GSC_HELPTEXTA|GCS_UNICODE. So, I simply test for GCS_HELPTEXTA and then test for the GCS_UNICODE bit. I also assume that if GetCommandString requests a Unicode string, then the max length of the string is half of what it would normally be. This may be incorrect, but it's a safe guess.

Future work

• There are 'extended' commands for the shell that are displayed when the user holds down the SHIFT key. For example, under NT, you'll get the additional commands 'Compress', 'Uncompress', and 'Open With...' (even if the file already has an association). I would like to add extended commands that only copy the filename(s) not the path (fewer keystrokes than F2, Ctrl-C, ESC). The documentation for IContextMenu::QueryContextMenu states that the CMF_EXTENDEDVERBS "...is set when the calling application wants extended verbs. Normal verbs are displayed when the user right-clicks an object. To display extended verbs, the user must right-click while pressing the SHIFT key." But, I could not get this to work nor find any examples. Let me know if you know how to do this.
• The bitmap for the icon gets XORed by the shell to make it look darker than it should. I've read that IContextMenu3 needs to be implemented to handle an ownerdraw menuitem in order to make the icons like as nice as the 'Send To' icons. A fix for this would be nice.
• Finally, I would like to put together an ATL Object wizard to help jumpstart the creation of these shell extensions. I'll do it if someone requests it.

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