public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
        {

        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            string s = this.shifer(tb.Text);
            tb2.Text = s;
        }

        private string shifer(string input)
        {
            string output = "";

            string buff,
            alfa = "абвгдеёжзийклмнопрстуфхцчшщъыьэюя",
            key = new string(alfa.ToCharArray().Reverse().ToArray());

            for (int i = 0; i < input.Length; i++)
            {
                if (!(input[i].Equals(' ') || input[i].Equals(',')) || input[i].Equals('.'))
                    output += key[alfa.IndexOf(input[i])];
                else
                    output += input[i];
            }

            return output;
        }
    }
}