Append to a text file in CSharp

using System;// w  ww . j  ava 2  s . c  om
using System.IO;
using System.Text;
using System.Collections.Generic;

class Program
{

    static void Main(string[] args)
    {

        string mydocpath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
        StringBuilder sb = new StringBuilder();

        foreach (string txtName in Directory.EnumerateFiles(mydocpath,"*.txt"))
        {
            using (StreamReader sr = new StreamReader(txtName))
            {
                sb.AppendLine(txtName.ToString());
                sb.AppendLine("line");
                sb.Append(sr.ReadToEnd());
                sb.AppendLine();
                sb.AppendLine();
            }

        }

        using (StreamWriter outfile = new StreamWriter(mydocpath + @"\AllTxtFiles.txt"))
        {
            outfile.Write(sb.ToString());
        }
    }
}