Appending the DataTable data to Text File in C# can be achieved easily by just lopping each row of the DataTable and placing to text file as code shown below. In the below code i have created the DataTable with three columns and entered the data. am passing the DataTable data to text file using  StreamWriter

DataTable to TextFile


using System;
using System.Web;
using System.IO;
using System.Data;

namespace WebApplication1
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Button1_Click(object sender, EventArgs e)
        {
            StreamWriter swExtLogFile = new StreamWriter("D:/Log/log.txt",true);
            DataTable dt = new DataTable();
           //Adding data To DataTable
            dt.Columns.Add("ID");
            dt.Columns.Add("Name");
            dt.Columns.Add("Address");
            dt.Rows.Add(1, "venki","Chennai");
            dt.Rows.Add(2, "Hanu","London");
            dt.Rows.Add(3, "john","Swiss");
           
            int i;
            swExtLogFile.Write(Environment.NewLine);
            foreach (DataRow row in dt.Rows)
            {  
                object[] array = row.ItemArray;
                for (i = 0; i < array.Length - 1; i++)
                {
                   swExtLogFile.Write(array[i].ToString() + " | ");
                }
                swExtLogFile.WriteLine(array[i].ToString());             
            }
            swExtLogFile.Write("*****END OF DATA****"+DateTime.Now.ToString());
            swExtLogFile.Flush();
            swExtLogFile.Close();

        }
    }
}