how to send mail using C# + asp.net

Question

MailMessage mail=new MailMessage ();
   //MailEncoding encod=new MailEncoding ();
   
   mail.From =TextBox1.Text; // accepting a value from textbox.
   mail.To =TextBox2.Text;
   mail.Subject =TextBox3.Text;
   mail.Body =TextBox4.Text ;
   mail.BodyFormat =MailFormat.Html ;
   String strdir="D:\\temp\\";
   String strfilename =Path.GetFileName (File1.PostedFile.FileName );
   File1.PostedFile.SaveAs (strdir+strfilename);
   mail.Attachments.Add (new MailAttachment (strdir+strfilename));

   try
   {
    SmtpMail.SmtpServer ="localhost";
    SmtpMail.Send (mail);
    
   }
   catch(Exception ex)
   {
    Response.Write("Exception Occured:    " +ex);   
   }
   finally
   {
    Response.Write("Your E-mail has been sent sucessfully");
   }

   con=new SqlConnection("user id=sa;password=rmax;data source=javastre-688yoi;database=master");
   con.Open ();  
   com=new SqlCommand ("insert into mail values('"+strdir+"')",con);
   com.ExecuteNonQuery ();
   Response.Write ("inserted");
 }

query:
1.how to attach a file and send it .
2.is the above program correct.
3.String strfilename =Path.GetFileName (File1.PostedFile.FileName ); how do we accept  value
from textbox and attach it.