(VB.NET UWP/WinRT) MIME Content-Transfer-Encoding Header Field

Explains the Content-Transfer-Encoding header field and how it affects how data is stored in the MIME.

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' The Content-Transfer-Encoding header field is typically set to one of these values:
' 
'   base64
'   quoted-printable
'   binary
'   8bit
'   7bit

' If the encoding is base64 or quoted-printable, then the bytes of the content are encoded as such.
' The values "8bit", "7bit", and "binary" all imply that NO encoding has been performed.

' Here are some guidelines and hints:
' 
' 1) "8bit", "7bit", and "binary" encodings (i.e. NO encoding) results in the smallest MIME.

' 2) A "binary" encoding implies non-text data.  Binary MIME CANNOT be retrieved as a string.  It can only be retrieved as a byte array.

' 3) "8bit" and "7bit" encodings imply text.  
'    7bit implies that the text is us-ascii (all byte values less than or equal to 0x7F).
'    8bit implies that the text contains non-us-ascii chars.

' 4) Base64 is the best choice for encoding non-text data, such as PDF's, images, etc.

' 5) quoted-printable is the best choice for encoding text data where most chars are going to be us-ascii. This would typically 
'    include all Western European languages.  For Asian, Arabic, Hebrew, etc. where most chars in the text are non-us-ascii,
'    the most efficient encoding would be base64.  

' Let's demonstrate with this small JPG image.
Dim success As Boolean
Dim sbJpgBase64 As New Chilkat.StringBuilder
sbJpgBase64.Append("/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g" & vbCrLf)
sbJpgBase64.Append("NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf" & vbCrLf)
sbJpgBase64.Append("YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj" & vbCrLf)
sbJpgBase64.Append("Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA" & vbCrLf)
sbJpgBase64.Append("AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY" & vbCrLf)
sbJpgBase64.Append("NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+" & vbCrLf)
sbJpgBase64.Append("pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq" & vbCrLf)
sbJpgBase64.Append("Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc" & vbCrLf)
sbJpgBase64.Append("enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg" & vbCrLf)
sbJpgBase64.Append("xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9" & vbCrLf)
sbJpgBase64.Append("DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA" & vbCrLf)
sbJpgBase64.Append("AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA" & vbCrLf)
sbJpgBase64.Append("QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx" & vbCrLf)
sbJpgBase64.Append("EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==" & vbCrLf)


Dim mime As New Chilkat.Mime
mime.ContentType = "image/jpeg"
mime.SetBodyFromEncoded("base64",sbJpgBase64.GetAsString())

Debug.WriteLine(mime.GetMime())
Debug.WriteLine("-")

' The result:
' 
' 	Content-Type: image/jpeg
' 	Content-Transfer-Encoding: base64
' 
' 	/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g
' 	NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf
' 	YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj
' 	Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA
' 	AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY
' 	NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+
' 	pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq
' 	Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc
' 	enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg
' 	xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9
' 	DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA
' 	AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA
' 	QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx
' 	EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==
' 


' Set the Content-Transfer-Encoding to "binary" by setting the Encoding property.
mime.Encoding = "binary"

' Try to get the MIME string.  This CANNOT be done.  The bytes of the JPG image do not
' represent chars, and to return a string means that bytes must be interpreted according
' to some character encoding (such as utf-8).  Non-text binary bytes can only be 
' contained in a string IF encoded in some way. Encodings such as Base64, quoted-printable,
' URL, etc. exist to make it possible to represent binary data in string format.   
Debug.WriteLine(mime.GetMime())
Debug.WriteLine("-")

' We CAN get the binary MIME as bytes..
Dim mimeBytes() As Byte
mimeBytes = mime.GetMimeBytes()

' Regardless of the Content-Transfer-Encoding, the 
' body content can always be retrieved and the body bytes
' decoded from whatever encoding is used..
Dim jpgBytes() As Byte
jpgBytes = mime.GetBodyBinary()

' To get the body in base64 format, first make sure
' the Content-Transfer-Encoding is base64, then call GetBodyEncoded.
mime.Encoding = "base64"
Dim jpgBase64 As String = mime.GetBodyEncoded()
Debug.WriteLine(jpgBase64)
Debug.WriteLine("-")

' Let's go back to "binary" MIME..
mime.Encoding = "binary"


' Let's say we have MIME, and it was loaded directly from a file, or from
' a byte array.  (It was not loaded from the contents of a string variable.)
' We don't know whether the MIME contains binary or 8bit encodings, and thus
' we dont' know if the MIME is safe to get as a string.
' The Convert8Bit method can be called to recursively traverse the MIME and set
' all 8bit or binary encodings to "base64".  This makes the MIME safe for storing in
' a string.
mime.Convert8Bit()
Dim mimeStr As String = mime.GetMime()
Debug.WriteLine(mimeStr)
Debug.WriteLine("-")