How Can I Add The Contents Of A Textbox And Datagridview Together In An Xml File?

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C#

XML

DataGridView

HI,
I want to add the contents of a textBox and DataGridView into an XML file together and then again load it back to the same.
I know how to add them separately but cannot achieve this.
This is how i was adding the datagridview and to the same i want to append the contents of the textbox

DataTable dt = new DataTable();
            foreach (DataGridViewColumn col in dgvIsland.Columns)
            {
                dt.Columns.Add(col.HeaderText);
            }
            foreach (DataGridViewRow row in dgvIsland.Rows)
            {
                DataRow dRow = dt.NewRow();
                foreach (DataGridViewCell cell in row.Cells)
                {
                    dRow[cell.ColumnIndex] = cell.Value;
                }
                dt.Rows.Add(dRow);
            }
            DataSet ds = new DataSet();
            ds.Tables.Add(dt);
            ds.WriteXml(filename);

Please help!!!
Thanks