stringbulder temp = new string builder;
foreach(datagridrow dgr in Datagridrows)
{
// this.dataGridView1.Rows[current row].Cells[current cell].Value
foreach(DataGridViewCell cell in dgr.Cell.Length)
{
temp.append(cell.Value);
}
}
StreamWriter sw=new StreamWriter("c:\\yourfile.txt", true, Encoding.ASCII);
string NextLine=temp.ToString();
sw.Write(NextLine);
sw.Close();