How to add new row in gridview and one column is auto generated value ex:S.No. and save to database

For Example

int nSlno = 0;
DataTable dt = new DataTable();
DataRow dr = null;
dt.Columns.Add(new DataColumn("RowNumber", typeof(string)));
dt.Columns.Add(new DataColumn("BrandId", typeof(int)));

dr = dt.NewRow();
dr["RowNumber"] = nSlno + 1;
dr["BrandId"] = "0";
dt.Rows.Add(dr);
ViewState["CurrentTable"] = dt;
Grid.DataSource = dt;
Grid.DataBind();