如何使用System.Net.Mail为电子邮件添加附件？

MailMessage mail = new MailMessage();

mail.From = new MailAddress("sender@example.com");
mail.To.Add("recipient@example.com");

mail.Subject = "邮件主题";
mail.Body = "邮件正文";

Attachment attachment = new Attachment("path/to/file.ext");
mail.Attachments.Add(attachment);

SmtpClient smtp = new SmtpClient("smtp.example.com");
smtp.Credentials = new NetworkCredential("username", "password");

smtp.Send(mail);

using System.Net.Mail;

MailMessage mail = new MailMessage();
mail.From = new MailAddress("sender@example.com");
mail.To.Add("recipient@example.com");
mail.Subject = "邮件主题";
mail.Body = "邮件正文";

Attachment attachment = new Attachment("path/to/file.ext");
mail.Attachments.Add(attachment);

SmtpClient smtp = new SmtpClient("smtp.example.com");
smtp.Credentials = new NetworkCredential("username", "password");
smtp.Send(mail);