C#中xml与数据库dataset的结合

加钮代码：

 private void button1_Click(object sender, EventArgs e).//按添
        {
            DataSet ds = new DataSet("students");
            DataTable dt = new DataTable("student");
            dt.Columns.Add("学号",typeof(string));
            dt.Columns.Add("姓名",typeof(string));
            dt.Columns.Add("专业",typeof(string));
            dt.Columns.Add("性别",typeof(string));
            DataRow dr = dt.NewRow();
            dr["学号"]=textBox1.Text;
            dr["姓名"]=textBox4.Text;
            dr["专业"]=textBox3.Text;
            dr["性别"]=textBox2.Text;
            ds.Tables.Add(dt);
            ds.Tables["student"].Rows.Add(dr);
            ds.WriteXml(@"../..students.xml");
        }

        private void button2_Click(object sender, EventArgs e)//读取
        {

            DataSet ds = new DataSet();
            ds.ReadXml(@"../..students.xml");
            dataGridView1.DataSource=ds.Tables["student"];
        }

 

问题是在添加之后在，students.xml中却未添加上，是为何啊求助啊