C# File

using System;
using System.IO;

class Test
{
    public static void Main()
    {
        string path = @"c:\temp\MyTest.txt";
        if (!File.Exists(path))
        {
            // Create a file to write to.
            using (StreamWriter sw = File.CreateText(path))
            {
                sw.WriteLine("Hello");
                sw.WriteLine("And");
                sw.WriteLine("Welcome");
            }
        }

        // Open the file to read from.
        using (StreamReader sr = File.OpenText(path))
        {
            string s = "";
            while ((s = sr.ReadLine()) != null)
            {
                Console.WriteLine(s);
            }
        }

            //copy sourcefile to targetfile
            File.Copy(sourcefile, targetfile, true);

           // update file content
            List<string> lineList = new List<string>();
            foreach (string line in File.ReadAllLines(ssMainFilename))
            {
                lineList.add(line);
            }

             File.WriteAllLines(ssMainFilename, lineList);
    }
}