(.NET Core C#) 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.
bool success;
Chilkat.StringBuilder sbJpgBase64 = new Chilkat.StringBuilder();
sbJpgBase64.Append("/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g\r\n");
sbJpgBase64.Append("NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf\r\n");
sbJpgBase64.Append("YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj\r\n");
sbJpgBase64.Append("Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA\r\n");
sbJpgBase64.Append("AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY\r\n");
sbJpgBase64.Append("NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+\r\n");
sbJpgBase64.Append("pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq\r\n");
sbJpgBase64.Append("Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc\r\n");
sbJpgBase64.Append("enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg\r\n");
sbJpgBase64.Append("xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9\r\n");
sbJpgBase64.Append("DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA\r\n");
sbJpgBase64.Append("AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA\r\n");
sbJpgBase64.Append("QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx\r\n");
sbJpgBase64.Append("EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==\r\n");

Chilkat.Mime mime = 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..
byte[] mimeBytes = null;
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..
byte[] jpgBytes = null;
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";
string jpgBase64 = 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();
string mimeStr = mime.GetMime();
Debug.WriteLine(mimeStr);
Debug.WriteLine("-");