ASP.NET 动态添加GridView空行方法

        protected DataTable InsertData()
        {
            DataTable dt = new DataTable("table1");
            dt.Columns.Add("ID");
            dt.Columns.Add("fanjianID");
            dt.Columns.Add("leaseArea");
            dt.Columns.Add("leasePrice");
            dt.Columns.Add("dailyRent");
            dt.Columns.Add("monthlyRent");
            for (int i = 0; i < GridView1.Rows.Count; i++)
           {
                GridViewRow row = GridView1.Rows[i];
                 DataRow newrow = dt.NewRow();
                 newrow[0] = GridView1.DataKeys[i].Value;
                 newrow[1] = ((DropDownList)row.FindControl("tb1")).SelectedValue;
                 newrow[2] = ((TextBox)row.FindControl("tb2")).Text;
                 newrow[3] = ((TextBox)row.FindControl("tb3")).Text;
                 newrow[4] = ((TextBox)row.FindControl("tb4")).Text;
                 newrow[5] = ((TextBox)row.FindControl("tb5")).Text;
                 dt.Rows.Add(newrow);
             }
             dt.AcceptChanges();
             return dt;
         }

         protected void btnAdd_Click(object sender, EventArgs e)
         {
             DataTable dt = this.InsertData();
             DataRow newrow = dt.NewRow();
             newrow["monthlyRent"] = " ";