ReplaceSubstring-in-File

using System;
using System.IO;
using System.Text;


class ReplaceSubstringsInFile
{
    static void Main()
    {
        StreamReader inputFile = new StreamReader("../../Input.txt");
        StreamWriter outputFile = new StreamWriter("../../Output.txt");

        using (inputFile)
        {
            using (outputFile)
            {
                string fileContent = inputFile.ReadToEnd();
                inputFile.Close();

                StringBuilder content = new StringBuilder(fileContent);
                content.Replace("start", "finish");

                outputFile.WriteLine(content);
            }
        }
    }
}