Export to Excel from Gridview using C#

string str = "your Query";
           ds = da.fill_data(str);
           DataTable dt = new DataTable();
           dt = ds.Tables[0];
           string attachment = "attachment; filename=ABC.xls";
           Response.ClearContent();
           Response.AddHeader("content-disposition", attachment);
           Response.ContentType = "application/vnd.ms-excel";
           string tab = "";
           foreach (DataColumn dc in dt.Columns)
           {
               Response.Write(tab + dc.ColumnName);
               tab = "\t";
           }
           Response.Write("\n");
           int i;
           foreach (DataRow dr in dt.Rows)
           {
               tab = "";
               for (i = 0; i < dt.Columns.Count; i++)
               {
                   Response.Write(tab + dr[i].ToString());
                   tab = "\t";
               }
               Response.Write("\n");
           }
           Response.End();