How to Write Text to a File

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;

namespace Write_Text_to_a_File
{
    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("Hi friends.......!");
                    sb.Append(sr.ReadToEnd());
                    sb.AppendLine();
                    sb.AppendLine();
                }
            }

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