Помогите найти ошибку в коде со StreamWriter

private void btSave_Click(object sender, EventArgs e)
        {
            SaveFileDialog sfd = new SaveFileDialog();
            sfd.ShowDialog();
            string s = sfd.FileName;
            StreamWriter sw = new StreamWriter(s, false, Encoding.UTF8);
            sw.WriteLine(this.studentslist.ToXML());
            sw.Close();
        }

using System.IO;

SaveFileDialog sfd = new SaveFileDialog();
if(sfd.ShowDialog()==DialogResult.OK)
{
         StreamWriter sw = new StreamWriter(sfd.FileName, false, Encoding.UTF8);
         sw.WriteLine(this.studentslist.ToXML());
         sw.Close();
}

var sfd = new SaveFileDialog();
if (sfd.ShowDialog()==DialogResult.OK)
    using(StreamWriter sw = new StreamWriter(sfd.FileName, false, Encoding.UTF8))
    {
         sw.WriteLine(this.studentslist.ToXML());
    }

using (var sfd = new SaveFileDialog())
   if (sfd.ShowDialog()==DialogResult.OK)
       using(StreamWriter sw = new StreamWriter(sfd.FileName, false, Encoding.UTF8))
          sw.WriteLine(this.studentslist.ToXML());