MIME Body vs. Sub-Parts

Explains the difference between sub-parts and body.

MIME documents (i.e. messages) can have a nested structure. The simplest MIME message contains a header followed by content (possibly encoded). The header is delimited from the content by two consecutive CRLF's.

A MIME message may be multipart. If so, then the Content-Type header field indicates "multipart" and the content the follows the header is itself a MIME message, which may also be multipart, etc. You can see how MIME messages effectively have a tree structure. The non-leaf nodes have Content-Types that are "multipart/" and the leaf nodes have non-multipart Content-Types.

The Chilkat MIME component/library uses two terms that need to be understood in order to effectively use the API. These are "body" and "sub-part". A multipart node has 1 or more sub-parts, but its body is empty. A leaf node (i.e. non-multipart node) has no sub-parts, but the body is (usually) non-empty.

This example creates a multipart MIME message and shows the results of getting the sub-parts and body of each node.

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Dim mime As New Chilkat.Mime()

Dim success As Boolean
success = mime.UnlockComponent("Anything for 30-day trial.")
If (success = false) Then
    MsgBox(mime.LastErrorText)
    Exit Sub
End If


'  Create a multipart/mixed MIME message with two sub-parts.
'  We'll use the Base64 encoding for the 2nd sub-part.
Dim part1 As New Chilkat.Mime()
part1.SetBodyFromPlainText("This is part 1")

Dim part2 As New Chilkat.Mime()
part2.SetBodyFromPlainText("This is part 2")
part2.Encoding = "base64"

mime.NewMultipartMixed()
mime.UseMmDescription = false
mime.AppendPart(part1)
mime.AppendPart(part2)

'  Show the MIME message:
TextBox1.Text = TextBox1.Text & mime.GetMime() & vbCrLf

'  Here's the MIME:
'  Content-Type: multipart/mixed;
'  	boundary="------------040605030407000302060008"
' 
'  --------------040605030407000302060008
'  Content-Type: text/plain;
'  	 charset="us-ascii"
'  Content-Transfer-Encoding: 7bit
' 
'  This is part 1
'  --------------040605030407000302060008
'  Content-Type: text/plain;
'  	 charset="us-ascii"
'  Content-Transfer-Encoding: base64
' 
'  VGhpcyBpcyBwYXJ0IDI=
' 
'  --------------040605030407000302060008--


'  If we examine the root node of the MIME message,
'  we see that it has 2 sub-parts and the body is empty (as expected)
Dim n As Long
n = mime.NumParts
TextBox1.Text = TextBox1.Text & "Num Parts = " _
     & n & vbCrLf
TextBox1.Text = TextBox1.Text & "Body = " & mime.GetBodyDecoded() & vbCrLf

'  The GetEntireBody retrieves the entire content after
'  the header.  (Perhaps GetEntireBody should've been named
'  GetEntireContent to make it less confusing...)
TextBox1.Text = TextBox1.Text & "---- EntireBody:" & vbCrLf
TextBox1.Text = TextBox1.Text & mime.GetEntireBody() & vbCrLf
TextBox1.Text = TextBox1.Text & "********" & vbCrLf

'  Now examine the 2nd sub-part.  It has a body encoded
'  using base64.  Get the contents of the body in both
'  decoded and encoded forms:
Dim part As Chilkat.Mime
'  index 0 is the 1st part, index 1 is the 2nd part.
part = mime.GetPart(1)
TextBox1.Text = TextBox1.Text & "Decoded Body:" & vbCrLf
TextBox1.Text = TextBox1.Text & part.GetBodyDecoded() & vbCrLf
TextBox1.Text = TextBox1.Text & "Encoded Body:" & vbCrLf
TextBox1.Text = TextBox1.Text & part.GetBodyEncoded() & vbCrLf