It’s technically possible, but difficult, to send an embedded OLE attachment using the MAPI controls. (OLE is a mechanism that allows users to create and edit documents containing items or “objects” created by multiple applications. In the context of sending email, an embedded OLE attachment is an OLE object—for example, a range of cells from a spreadsheet—that is transmitted along with the message. A general discussion of OLE is not within the scope of this book; consult Microsoft documentation for further information about this technology.)
If you need to send embedded OLE attachments, you’re better off using Collaboration Data Objects (CDO). However, I’ll explain briefly how it could be done with the MAPI controls.
As explained in the previous section, sending a file attachment
requires setting the AttachmentType property of the
MAPIMessages control to mapData. There are two
other legal values for this property: mapEOLE and
mapSOLE. Either value can be used to send an OLE
object. The difference is that mapEOLE is for an
“editable” OLE object, and mapSOLE is
for a “static” (i.e., non-editable) OLE object.
On the receiving end, the mail client displays the OLE object within
the body of the message. If the AttachmentType property was set to
mapEOLE, the user can double-click the object to
begin editing it. For this to work, the user must have software on
his or her system that knows how to edit the kind of object that was
sent. For example, if you send an OLE object that contains a range of
cells from a Microsoft Excel spreadsheet, the receiving user must
have Excel in order to edit the object. However, it is not necessary
to have Excel simply to view the object. (OLE objects carry inside
them a static visual representation of their data.)
To attach the OLE object to an outgoing message, you must
serialize the object into a file, and set the
AttachmentPathName property of the
MAPIMessages control to the fully qualified path of the file.
Serialization is the process of copying an object’s state to
some kind of sequential storage device, such as a file. Copying to a
non-volatile storage device, such as a file, is also known as
persisting the object. The final step is to set
the AttachmentName property of the MAPIMessages control equal to the
name of the object’s class—for example,
"Excel.Sheet“.
The hard part of all this, of course, is serializing the OLE object.
The MAPI controls provide no facility for doing this, so you’re
forced to call into the Windows API. While I won’t go into
details, the basic idea is to open the file as an OLE stream and pass
the IStream interface pointer to the
OleSaveAsStream API. As I mentioned before, if
you’re going to these lengths, you’re better off making
the jump to CDO.
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