string[] rowString = new string[dataSet11.Tables["Customers"].Rows.Count];
int i=0;
foreach (DataRow dr in dataSet11.Tables["Customers"].Rows)
{
    rowString[i] = dr.ItemArray.GetValue(0).ToString() + ",";
    rowString[i] += dr.ItemArray.GetValue(1).ToString() + ",";
    rowString[i] += dr.ItemArray.GetValue(2).ToString();
    i++;
}
File.WriteAllLines(@"C:\customer.txt", rowString);
MessageBox.Show("Data Exported to text file");