Create an EML file in C# · onemail cookbook

Build the message from MimeMultipart and MimePart, add the headers, and call WriteTo. The program below writes an email with plain text and HTML bodies and a CSV attachment, then reads it back.

Terminal

dotnet new console -o CreateEml && cd CreateEml
dotnet add package RecursiveAI.OneMail --version 0.2.0-preview.2

Put this in Program.cs and run dotnet run:

using System.Globalization;
using System.Text;
using RecursiveAI.OneMail;
using RecursiveAI.OneMail.Mime;
using RecursiveAI.OneMail.Mime.Transfer;
using RecursiveAI.OneMail.Text;
 
var message = new MimeMultipart("mixed");
// Address headers are written as given: encode a non-ASCII display name yourself
message.Headers.Add("From", $"{Rfc2047.Encode("Zoë Martín")} <[email protected]>");
message.Headers.Add("To", "Sam Lee <[email protected]>");
// Free text such as the subject is encoded for you
message.Headers.Add("Subject", "Q3 report – final");
string date = DateTime.UtcNow.ToString("ddd, dd MMM yyyy HH:mm:ss +0000",
    CultureInfo.InvariantCulture);
message.Headers.Add("Date", date);
message.Headers.Add("Message-ID", $"<{Guid.NewGuid():N}@example.com>");
message.Headers.Add("MIME-Version", "1.0");
 
// Plain text and HTML versions of the same body
var body = new MimeMultipart("alternative");
var text = new MimePart("text/plain; charset=utf-8");
text.SetDecodedContent(Utf8("Hi Sam,\r\n\r\nThe report is attached.\r\n"),
    ContentTransferEncoding.QuotedPrintable);
var html = new MimePart("text/html; charset=utf-8");
html.SetDecodedContent(Utf8("<p>Hi Sam,</p><p>The report is attached.</p>"),
    ContentTransferEncoding.QuotedPrintable);
body.Add(text);
body.Add(html);
message.Add(body);
 
// An attachment, base64-encoded (the default)
var csv = new MimePart("text/csv; name=\"q3.csv\"");
csv.Headers.Add("Content-Disposition", "attachment; filename=\"q3.csv\"");
csv.SetDecodedContent(Utf8("month,revenue\r\nJul,1200\r\nAug,1350\r\n"));
message.Add(csv);
 
using (FileStream file = File.Create("report.eml"))
    message.WriteTo(file);
Console.WriteLine($"report.eml: {new FileInfo("report.eml").Length:N0} bytes");
 
// Read it back
IMailItem check = Mail.Parse(File.ReadAllBytes("report.eml"));
Console.WriteLine($"{check.From[0]} -> {check.ToRecipients[0]}: {check.Subject}");
string plain = check.TextBody!.ReadText();
Console.WriteLine($"text: {plain.ReplaceLineEndings(" ").Trim()}");
IMailAttachment attached = check.Attachments[0];
Console.WriteLine($"attachment: {attached.FileName}, {attached.Size} bytes");
Console.WriteLine($"defects: {check.Defects.Count()}");
 
static byte[] Utf8(string s) => Encoding.UTF8.GetBytes(s);

Text

report.eml: 1,019 bytes
Zoë Martín <[email protected]> -> Sam Lee <[email protected]>: Q3 report – final
text: Hi Sam,  The report is attached.
attachment: q3.csv, 35 bytes
defects: 0

report.eml opens in any mail program; Python’s email package also reads it with no defects. A boundary for each multipart is generated for you, and lines end in CRLF as MIME requires.

How the parts fit

multipart/mixed holds the body and the attachments.
multipart/alternative inside it holds versions of the same body, plainest first. Mail programs show the last one they support.
SetDecodedContent takes the raw bytes and encodes them: base64 by default, or quoted-printable for text people may read in the source.
Header values must be one line. A line break throws, so user input can’t inject headers.

Limits

A preview. 0.2.0-preview.2: the API can change before 1.0.
A low-level API. There is no message builder yet. You set each header yourself, and you encode non-ASCII display names in address headers with Rfc2047.Encode, as above. Subjects and other free text are encoded for you.
No sending. SMTP isn’t in this preview. The file is ready to hand to any SMTP client.
No Outlook formats. OneMail can’t write MSG or PST files yet.

Reading messages back: Parse an EML file. The writer: Reading and writing MIME.