How to generate and parse MIME messages with alternative content types (e.g., text and HTML) in Go?

To generate and parse MIME messages with alternative content types in Go, you can use the "mime/multipart" and "mime/textproto" packages. Here's an example code that demonstrates the process:

Generating a MIME message:

package main

import (
	"bytes"
	"fmt"
	"io/ioutil"
	"mime/multipart"
	"net/mail"
	"net/textproto"
)

func main() {
	// Create a buffer to store the message
	var message bytes.Buffer

	// Create a new multipart writer
	writer := multipart.NewWriter(&message)

	// Set the content type of the multipart message
	writer.SetBoundary("MyBoundary")

	// Create the first message part with text/plain content type
	part1 := make(textproto.MIMEHeader)
	part1.Set("Content-Type", "text/plain")

	// Write the content of the first message part
	part1Writer, _ := writer.CreatePart(part1)
	part1Writer.Write([]byte("This is the text part."))

	// Create the second message part with text/html content type
	part2 := make(textproto.MIMEHeader)
	part2.Set("Content-Type", "text/html")

	// Write the content of the second message part
	part2Writer, _ := writer.CreatePart(part2)
	part2Writer.Write([]byte("<p>This is the HTML part.</p>"))

	// Close the multipart writer after writing all parts
	writer.Close()

	// Print the generated MIME message
	fmt.Println(message.String())
}

Parsing a MIME message:

package main

import (
	"fmt"
	"io"
	"mime"
	"mime/multipart"
	"net/mail"
)

func main() {
	// Example MIME message
	message := `Content-Type: multipart/alternative; boundary=MyBoundary

--MyBoundary
Content-Type: text/plain

This is the text part.
--MyBoundary
Content-Type: text/html

<p>This is the HTML part.</p>
--MyBoundary--`

	// Parse the message
	parsedMessage, _ := mail.ReadMessage(bytes.NewBufferString(message))

	// Get the content type of the message
	contentType := parsedMessage.Header.Get("Content-Type")

	// Check if the message is multipart
	isMultipart := false
	mediatype, params, err := mime.ParseMediaType(contentType)
	if err == nil && mediatype == "multipart/alternative" {
		isMultipart = true
	}

	if isMultipart {
		// Create a new multipart reader using the boundary specified in the content type
		multipartReader := multipart.NewReader(parsedMessage.Body, params["boundary"])

		// Iterate over each part in the multipart message
		for {
			part, err := multipartReader.NextPart()
			if err == io.EOF {
				break
			}

			// Get the content type of the part
			partContentType := part.Header.Get("Content-Type")

			// Check if the part is text/plain
			if partContentType == "text/plain" {
				// Read and print the content of the part
				partContent, _ := ioutil.ReadAll(part)
				fmt.Println("Text Part:", string(partContent))
			}

			// Check if the part is text/html
			if partContentType == "text/html" {
				// Read and print the content of the part
				partContent, _ := ioutil.ReadAll(part)
				fmt.Println("HTML Part:", string(partContent))
			}
		}
	}
}

This example demonstrates how to generate a MIME message with text and HTML parts, as well as how to parse such a message to retrieve the content of each part.