HTMLEncoder.cs

using System;
using System.Windows;
using System.IO;
using System.Text.RegularExpressions;
using Microsoft.Win32;

namespace HTMLEncoder
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void btnOpenFile_Click(object sender, RoutedEventArgs e)
        {
            OpenFileDialog openFileDialog = new OpenFileDialog();
            openFileDialog.ShowDialog();
            string file = openFileDialog.FileName;

            if (!(String.IsNullOrEmpty(file)))
            {
                string fileStr = File.ReadAllText(file);

                fileStr = Regex.Replace(fileStr, " & ", " &amp; ");

                fileStr = Regex.Replace(fileStr, "ä", "&auml;");
                fileStr = Regex.Replace(fileStr, "ö", "&ouml;");
                fileStr = Regex.Replace(fileStr, "ë", "&euml;");
                fileStr = Regex.Replace(fileStr, "ï", "&iuml;");
                fileStr = Regex.Replace(fileStr, "ü", "&uuml;");

                fileStr = Regex.Replace(fileStr, "Ä", "&Auml;");
                fileStr = Regex.Replace(fileStr, "Ö", "&Ouml;");
                fileStr = Regex.Replace(fileStr, "Ë", "&Euml;");
                fileStr = Regex.Replace(fileStr, "Ï", "&Iuml;");
                fileStr = Regex.Replace(fileStr, "Ü", "&Uuml;");

                File.WriteAllText(file, fileStr);

                MessageBox.Show("Contents replaced succesfully");
            }
        }
    }
}