C# Signature:

  /// <summary>
  /// The MAPISendMail function sends a message.
  ///
  /// This function differs from the MAPISendDocuments function in that it allows greater
  /// flexibility in message generation.
  /// </summary>
  [DllImport("MAPI32.DLL", CharSet=CharSet.Ansi)]
  public static extern uint MAPISendMail(IntPtr lhSession, IntPtr ulUIParam,
    MapiMessage lpMessage, uint flFlags, uint ulReserved);

VB Signature:

    <DllImport("MAPI32.DLL")> _
    Private Shared Function MAPISendMail(ByVal sess As IntPtr, ByVal hwnd As IntPtr, ByVal message As MapiMessage,
       ByVal flg As Integer, ByVal rsv As Integer) As Integer
    End Function

User-Defined Structures:

MAPI

User-Defined Constants:

MAPI

Notes:

This signature is valid, if the MapiMessage is defined as a class. If MapiMessage is defined as a structure a 'ref' has to be added before 'MapiMessage lpMessage' parameter

Tips & Tricks:

Watch it! MAPI32 is not supported from managed code [http://blogs.msdn.com/mstehle/archive/2007/10/03/fyi-why-are-mapi-and-cdo-1-21-not-supported-in-managed-net-code.aspx]

Please add some!

Sample Code:

    [DllImport("MAPI32.DLL")]
    static extern int MAPISendMail(IntPtr sess, IntPtr hwnd, MapiMessage message, int flg, int rsv);

    public int SendMail(string fileName)
    {
        MapiMessage msg = new MapiMessage();
        msg.subject = "Hello World!";
        msg.noteText = "See attached file for details";
        msg.files = GetAttachments(fileName, out msg.fileCount);

        int result = MAPISendMail(new IntPtr(0), new IntPtr(0), msg, 0x00000001 | 0x00000008, 0);
        if (result > 1 || result < 0)
        throw new System.InvalidOperationException();
        return result;
    }

    IntPtr GetAttachments(string fileName, out int fileCount)
    {
        int size = Marshal.SizeOf(typeof(MapiFileDesc));
        IntPtr intPtr = Marshal.AllocHGlobal(size);

        MapiFileDesc mapiFileDesc = new MapiFileDesc();
        //An integer used to indicate where in the message text to render the attachment.
        mapiFileDesc.position = -1;
        int ptr = (int)intPtr;

        mapiFileDesc.name = Path.GetFileName(fileName);
        mapiFileDesc.path = fileName;
        Marshal.StructureToPtr(mapiFileDesc, (IntPtr)ptr, false);
        ptr += size;

        fileCount = 1;
        return intPtr;
    }
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
    public class MapiMessage
    {
    public int reserved;
    public string subject;
    public string noteText;
    public string messageType;
    public string dateReceived;
    public string conversationID;
    public int flags;
    public IntPtr originator;
    public int recipCount;
    public IntPtr recips;
    public int fileCount;
    public IntPtr files;
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
    public class MapiFileDesc
    {
    public int reserved;
    public int flags;
    public int position;
    public string path;
    public string name;
    public IntPtr type;
    }

Alternative Managed API:

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