Enviar correo con fichero adjunto

codesendmail.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Net.Mail;
using System.Net.Mime;

namespace send_mail
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnEnviarMail_Click(object sender, EventArgs e)
        {
            MailMessage mail = new MailMessage();
            SmtpClient SmtpServer = new SmtpClient("smtp.office365.com");
            mail.From = new MailAddress("your_username@...com");
            mail.To.Add("your_username@...com");
            mail.Subject = "Prueba de correo";
            mail.Body = "Correo con mensaje adjunto!";

            System.Net.Mail.Attachment attachment;
            attachment = new System.Net.Mail.Attachment("c:/test.txt");
            mail.Attachments.Add(attachment);

            SmtpServer.Port = 587;
            SmtpServer.Credentials = new System.Net.NetworkCredential("your_username", "your_password");
            SmtpServer.EnableSsl = true;

            SmtpServer.Send(mail);
           
        }
    }
}

Gracias