SendTo mail recipient

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This document provides a code snippet in order to programmatically send one or more files to the mail recipient, mimicing the SendTo mail recipient shell extension. I have heard many people searching for this feature, and actually I thought that, as there doesn't seem to be such source code posted on the net, I could just as well post it.

1. Introducing the SendTo mail shortcut

You may skip this section if you are not interested in the r.e. technique.

The SendTo mail shortcut is a shell extension. See Mike Dunn's complete idiot guide[^] for further info.

The trick is to find out that the SendTo mail recipient shortcut is actually essentially an empty file with .MAPIMAIL as (hidden) extension name. Then, by looking up file type association in the registry (HKCR\.MAPIMAIL), it's straight forward to figure out that it is targeting a COM object with clsid = {9E56BE60-C50F-11CF-9A2C-00A0C90A90CE}. This object is sendmail.dll, a COM helper which takes advantage of either Outlook Express or Outlook to send email file attachments. Next, looking up this CLSID in OLE View clearly showed that the sendmail COM component also implements the IDropTarget, IShellExtInit and IPersistFile interfaces, just like any drop handler[^].

Ok, basically I need to prepare a bag with dropped filenames, make sure they can be retrieved by implementing the IDataObject interface, a simple communication interface, and then mimic a standard drag and drop sequence, with two check points : DragEnter(IDataObject*) and Drop(IDataObject*).

One of the interesting points is to start implementing the IDataObject interface starting with contract, ie the methods it is supposed to expose. And then cowardly insert a breakpoint into any default method implementation only to get to know whether it's called or not, and in what order.

If you are interested in mimicing other SentTo shortcuts, don't hesitate to check out this registry key : HKLM \ Software \ Microsoft \ Windows \ CurrentVersion \ Shell Extensions \ Approved.

2. IDataObject implementation

The following code implements the IDataObject interface. In fact, only a few methods are required to be implemented. Those are, in order :

• EnumFormatEtc(), called by the sendmail helper to know what content formats the IDataObject is holding
• IEnumFORMATETC::Next(),Reset(), called to list all formats. We are expected to let the sendmail helper know that we do hold the CF_HDROP clipboard format (standard used for file drag-and-drop support), even if in fact we are not using the clipboard at all.
• GetData(), called to actually get the list of files we are willing to send.

Because on Windows file drag-and-drop operations rely on the CF_HDROP / DROPFILES structure, we simply prepare such a structure to play with. Here is the code :

#include <windows.h>
#include <ole2.h> // IDataObject
#include <shlobj.h> // DROPFILES
#include <tchar.h> // TCHAR

class CDataObject : public IDataObject, IEnumFORMATETC
{
  // Members
protected:
  BOOL m_bReset;
  LPTSTR m_szFiles;
  int m_nLen;

	// Constructor
public:
  CDataObject(LPTSTR szFiles)
  {
    Reset();
		
    if (!szFiles)
    {
      m_szFiles = NULL;
      return;
    }

    // replace \n chars with \0 chars
    m_nLen = _tcslen(szFiles)+1;
    m_szFiles = new TCHAR[m_nLen];
    memcpy(m_szFiles, szFiles, m_nLen * sizeof(TCHAR));

    LPTSTR szTmp = m_szFiles;
    while ( szTmp=_tcschr(szTmp,'\n') )
      *szTmp++ = '\0';

  }

  virtual ~CDataObject()
  {
    delete [] m_szFiles;
  }

public:
  HRESULT __stdcall QueryInterface(REFIID iid, void** ppvObject)
  {
    *ppvObject = (IDataObject*) this;
    return S_OK;
  }

  ULONG __stdcall AddRef()
  {
    return 1;
  }
  ULONG __stdcall Release()
  {
    return 0;
  }

	

  // IDataObject implementation
  //
  HRESULT __stdcall GetData(FORMATETC* pFormatetc, STGMEDIUM* pmedium)
  {
    if (pFormatetc->cfFormat != CF_HDROP  || !pmedium)
      return S_FALSE;

    if (!m_szFiles)
      return S_FALSE; // make sure to set the files before

    pmedium->tymed = TYMED_HGLOBAL;

    // set DROPFILES structure
    HGLOBAL hglbCopy = ::GlobalAlloc(GMEM_MOVEABLE, 
                 sizeof(DROPFILES) + (m_nLen + 2) * sizeof(TCHAR)); 
    LPDROPFILES pDropFiles = (LPDROPFILES) ::GlobalLock(hglbCopy);
    pDropFiles->pFiles = sizeof(DROPFILES);
    pDropFiles->pt.x = pDropFiles->pt.y = 0;
    pDropFiles->fNC = TRUE;
    pDropFiles->fWide = FALSE; // ANSI charset
    LPTSTR lptstrCopy = (LPTSTR) pDropFiles;
    lptstrCopy += pDropFiles->pFiles;
    memcpy(lptstrCopy, m_szFiles, m_nLen * sizeof(TCHAR)); 
    lptstrCopy[m_nLen] =  '\0';    // null character 
    lptstrCopy[m_nLen+1] = '\0';    // null character 
    ::GlobalUnlock(hglbCopy); 

    pmedium->hGlobal = hglbCopy;
    pmedium->pUnkForRelease = NULL;

    return S_OK;
  }

  HRESULT __stdcall GetDataHere(FORMATETC* pFormatetc, STGMEDIUM* pmedium)
  {
    return S_OK;
  }

  HRESULT __stdcall QueryGetData(FORMATETC* pFormatetc)
  {
    return S_OK;
  }

  HRESULT __stdcall GetCanonicalFormatEtc(FORMATETC* pFormatetcIn, 
                                          FORMATETC* pFormatetcOut)
  {
    return S_OK;
  }

  HRESULT __stdcall SetData(FORMATETC* pFormatetc, STGMEDIUM* pmedium, BOOL fRelease)
  {
    return S_OK;
  }

  HRESULT __stdcall EnumFormatEtc(DWORD dwDirection, IEnumFORMATETC** ppenumFormatetc)
  {
    if (dwDirection==DATADIR_GET)
    {
      *ppenumFormatetc = this;
      return S_OK;
    }
    else
      return S_FALSE;
  }

  HRESULT __stdcall DAdvise(FORMATETC* pFormatetc, 
                            DWORD advf, 
                            IAdviseSink* pAdvSink, 
                            DWORD* pdwConnection)
  {
    return S_OK;
  }

  HRESULT __stdcall DUnadvise(DWORD dwConnection)
  {
    return S_OK;
  }

  HRESULT __stdcall EnumDAdvise(IEnumSTATDATA** ppenumAdvise)
  {
    return S_OK;
  }


  // IEnumFORMATETC implementation
  //
  HRESULT __stdcall Next( 
            /*[in]*/ ULONG celt,
            /*[out]*/ FORMATETC __RPC_FAR* rgelt,
            /*[out]*/ ULONG __RPC_FAR* pceltFetched)
  {
    if (!m_bReset) return S_FALSE;

    m_bReset = FALSE;

    FORMATETC fmt;
    fmt.cfFormat = CF_HDROP;
    fmt.dwAspect = DVASPECT_CONTENT;
    fmt.lindex = -1;
    fmt.ptd = NULL;
    fmt.tymed = TYMED_HGLOBAL;
    *rgelt = fmt; // copy struct
    if (pceltFetched) *pceltFetched = 1;

    return S_OK;
  }
        
  HRESULT __stdcall Skip(/*[in]*/ ULONG celt)
  {
    return S_FALSE;
  }
        
  HRESULT __stdcall Reset()
  {
    m_bReset = TRUE;
    return S_OK;
  }
        
  HRESULT __stdcall Clone( 
            /* [out] */ IEnumFORMATETC** ppenum)
  {
    return S_OK;
  }
};

3. Using it

  ::CoInitialize(NULL);

  CDataObject cdobj("c:\\346.jpg\nC:\\tmp\\myfile.zip");
  IDataObject *pDataObject = &cdobj;


  IDropTarget *pDropTarget = NULL;

  // create an instance, and mimic drag-and-drop
  hr = ::CoCreateInstance( CLSID_SendMail, 
                         NULL, CLSCTX_ALL,
                         IID_IDropTarget, 
                         (void **)&pDropTarget);
  if (SUCCEEDED(hr))
  {
    POINTL pt = {0,0};
    DWORD dwEffect = 0;
    pDropTarget->DragEnter(pDataObject, MK_LBUTTON, pt, &dwEffect);
    pDropTarget->Drop(pDataObject, MK_LBUTTON, pt, &dwEffect);

    ::Sleep(6*1000);

    pDropTarget->Release();
  }

  ::CoUninitialize();

To compile this code, you only need WIN32.

4. why SendTo is better than the mailto trick

You could tell me that shellexecuting a mailto url is just as fine, and much simpler code in practice. Yes and no. Yes, it does so, and it can't be simpler since that's only one line of code (just remember to escape ascii chars in the body with hexa replacements of the form %0D, %20, ...).

No, it does not provide support for file attachment(s), which is why the SendTo shortcut comes handy. And there is more to it. The truth is that ::ShellExecute() cannot handle parameter strings over 2048 bytes, which means your email body size cannot go beyond 2048 bytes. If you try to send a large email, it will result in a GPF. At this point, you could replace ::ShellExecute with a well thought ::WinExec call using the actual mailto command line declared in the registry and target the current email client (for instance, "%ProgramFiles%\Outlook Express\msimn.exe" /mailurl:%1), but then the limitation is 32KB. As a conclusion, there is no way to send emails larger than 32KB using the mailto protocol. The SendTo shortcut explained in this article is definitely the way to go !