MainWindow.xaml.cs

using System;
using System.Windows;
using System.Windows.Input;
using System.Windows.Threading;
using Microsoft.Win32;

namespace MusicPlayer
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private readonly Backend _player;
        private readonly DispatcherTimer _timer;

        public MainWindow()
        {
            InitializeComponent();
            _player = new Backend();
            Playlist.ItemsSource = _player.Playlist;
            _timer = new DispatcherTimer {Interval = new TimeSpan(0, 0, 0, 0, 500)};
            _timer.Tick += Update;
            _timer.Start();
        }

        private void MenuItem_Click(object sender, RoutedEventArgs e)
        {
            Main.Close();
        }

        private void AddButton_Click(object sender, RoutedEventArgs e)
        {
            var openFiles = new OpenFileDialog {Multiselect = true};
            openFiles.ShowDialog();
            string[] files = openFiles.FileNames;
            foreach (string file in files)
                _player.Playlist.Add(new Song(file));
            Playlist.ItemsSource = _player.Playlist;
        }

        private void PlayButton_Click(object sender, RoutedEventArgs e)
        {
            _player.Play();
            PauseButton.Visibility = Visibility.Visible;
        }

        private void nextButton_Click(object sender, RoutedEventArgs e)
        {
            _player.Next();
        }

        private void PreviousButton_Click(object sender, RoutedEventArgs e)
        {
            _player.Previous();
        }

        private void PauseButton_Click(object sender, RoutedEventArgs e)
        {
            _player.Pause();
            PauseButton.Visibility = Visibility.Hidden;
        }

        private void stopButton_Click(object sender, RoutedEventArgs e)
        {
            _player.Stop();
            PauseButton.Visibility = Visibility.Hidden;
        }

        private void Update(object sender, EventArgs e)
        {
            if (_player.Player.NaturalDuration.HasTimeSpan)
                ProgressBar.Value = _player.Player.Position.TotalSeconds/
                                    _player.Player.NaturalDuration.TimeSpan.TotalSeconds*10;
        }

        private void Playlist_MouseDoubleClick(object sender, MouseButtonEventArgs e)
        {
            _player.ChangeTrack(Playlist.SelectedIndex);
        }
    }
}