C# - cannot attach file to an email [duplicate]

Question

This question already has an answer here:

• Attachment filename not en/de-coded correctly when longer than 13 characters 1 answer

I have a service that creates an Excel file and sends it by email.

This service works correctly on a test server but on a production server I have a bug - there is strange file in attachments without extension with name like this =utf-8B0J3QtdC0YDQvtGB0YLQsNCy0LvQtdC9 and the same content inside instead of an Excel file.

There is my sending method. I suppose that there are no troubles in my code.

public static void SendEmail(string subject, string body, string recipients, Attachment attachment = null)
    {
        string mailServer = Settings.Default.MailServer;

        MailMessage message = new MailMessage(Settings.Default.MailSender, recipients);
        foreach (string address in Settings.Default.MailCopyTo.Split(',').Select(x => x.Trim()))
        {
            if (!string.IsNullOrEmpty(address))
                message.CC.Add(new MailAddress(address));
        }
        message.IsBodyHtml = true;
        message.Body = body;
        message.Subject = subject;

        if(attachment != null)
            message.Attachments.Add(attachment);


        SmtpClient client = new SmtpClient(mailServer);
        var AuthenticationDetails = new NetworkCredential(Settings.Default.MailLogin, Settings.Default.MailPassword);
        client.Credentials = AuthenticationDetails;
        client.Send(message);
    }

Answer 1

There's a bug in the .NET Framework with attachments the file name of which is longer than 41 unicode characters.

I also had the same problem and fixed it by truncating the file name to 40 characters. What I did:

1. Get file extension
2. Get file name without extension
3. Shorten file name to 40 - extension.Length characters
4. Add extension

I quote from an answer in an MSDN thread :

Yes we are aware of this problem and have already addressed it for future releases. [...]

In .NET 4.0 SmtpClient now implements the RFC line length limits (76 chars). This required extensive changes to the encoding process and we have uncovered a few issues like the one you describe.

In your case, yes, there is an issue with non-ascii attachment names that would be encoded to more than 41 utf-8 bytes (Encoding.UTF8.GetByteCount(fileName);). In this scenario the names are encoded twice and may have extra line breaks in them. The only known workaround is to limit the length of non-ascii file names.