foreach (DataRow dr in dt.Rows)
{
    for (int i = 0; i < iColCount; i++)
    {
        if (!Convert.IsDBNull(dr[i]))
        {
            sw.Write(dr[i].ToString());
        }

        if (i < iColCount - 1)
        {
            sw.Write("  ");
        }
    }

    sw.Write(sw.NewLine);
}