write in text file in asp.net

 protected void imgyellow_Click(object sender, ImageClickEventArgs e)
        {
            StreamWriter writer;
            string txtfilepath = Server.MapPath("../csspath.txt");
            using (writer = new StreamWriter(txtfilepath))
            {
                writer.WriteLine("~/CSS/calculator.css,~/CSS/ddsmoothmenu.css");
                writer.Close();
            }        
        }