Getting Thumbnail from Video using MediaPlayer Class in WPF

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Collections.ObjectModel;
using System.IO;
using System.Threading;
using System.Threading.Tasks;

namespace VideoScreenshot
{
    public partial class MainWindow : Window
    {
        ObservableCollection<MediaItem> thumbnailCollection = new ObservableCollection<MediaItem>();
        ObservableCollection<int> speedList = new ObservableCollection<int>();
        ObservableCollection<int> positionList = new ObservableCollection<int>();

        int thumbnailsInProgress = 0;

        public MainWindow()
        {
            InitializeComponent();
            mediaListBox.ItemsSource = thumbnailCollection;
            for (int i = 0; i < 20; i++)
            {
                speedList.Add(i + 1);
                positionList.Add(i);
            }
            waitTimeBox.ItemsSource = speedList;
            positionBox.ItemsSource = positionList;
            waitTimeBox.SelectedIndex = 3;
            positionBox.SelectedIndex = 1;
        }

        private void importMediaButton_Click(object sender, RoutedEventArgs e)
        {
            Microsoft.Win32.OpenFileDialog ofd = new Microsoft.Win32.OpenFileDialog();
            ofd.CheckPathExists = true;
            ofd.Multiselect = true;
            bool? result = ofd.ShowDialog();
            if (result == true)
            {  
                foreach (string str in ofd.FileNames)
                {
                    int waitTime = (int)waitTimeBox.SelectedValue;
                    int position = (int)positionBox.SelectedValue;
                    if (thumbnailsInProgress++ == 0)
                    {
                        animationGrid.Visibility = System.Windows.Visibility.Visible;
                        downloadAnimation.StartAnimation();
                    }

                    //Since the Task.Factory.StartNew() will be called after the loop has already exited, 
                    //we need to create a copy of 's'; otherwise the same value will be passed to multiple invocations!!
                    string fileName = str;
                    Task.Factory.StartNew(() => { ImportMediaThreadProc(fileName, waitTime, position); });
                }
            }
        }
        
        //We would like to create thumbnails one after the other; so we will use a lock to give 
        //access to only one path of execution to this method
        object importMediaLock = new object();
        void ImportMediaThreadProc(string mediaFile, int waitTime, int position)
        {
            lock (importMediaLock)
            {
                MediaPlayer player = new MediaPlayer { Volume = 0, ScrubbingEnabled = true };
                player.Open(new Uri(mediaFile));
                player.Pause();
                player.Position = TimeSpan.FromMilliseconds(position * 1000);
                //We need to give MediaPlayer some time to load. The efficiency of the MediaPlayer depends                 
                //upon the capabilities of the machine it is running on
                System.Threading.Thread.Sleep(waitTime * 1000);

                //120 = thumbnail width, 90 = thumbnail height and 96x96 = horizontal x vertical DPI
                //An in actual application, you wouldn's probably use hard coded values!
                RenderTargetBitmap rtb = new RenderTargetBitmap(120, 90, 96, 96, PixelFormats.Pbgra32);
                DrawingVisual dv = new DrawingVisual();
                using (DrawingContext dc = dv.RenderOpen())
                {
                    dc.DrawVideo(player, new Rect(0, 0, 120, 90));
                }
                rtb.Render(dv);
                Duration duration = player.NaturalDuration;
                int videoLength = 0;
                if (duration.HasTimeSpan)
                {
                    videoLength = (int)duration.TimeSpan.TotalSeconds;
                }
                BitmapFrame frame = BitmapFrame.Create(rtb).GetCurrentValueAsFrozen() as BitmapFrame;
                BitmapEncoder encoder = new JpegBitmapEncoder();
                encoder.Frames.Add(frame as BitmapFrame);

                //We cannot create the thumbnail here, as we are not in the UI thread right now
                //It is the responsibility of the calee to close the MemoryStream
                //We will instead call a method which will do its stuff on the UI thread!
                MemoryStream memoryStream = new MemoryStream();
                encoder.Save(memoryStream);
                CreateMediaItem(memoryStream, mediaFile, videoLength);
                player.Close();
            }
        }

        void CreateMediaItem(MemoryStream ms, string name, int duration)
        {
            Dispatcher.Invoke(new Action(delegate
            {
                ms.Position = 0;
                MediaItem mediaItem = new MediaItem();
                BitmapImage bitmapImage = new BitmapImage();
                bitmapImage.BeginInit();
                bitmapImage.CacheOption = BitmapCacheOption.OnLoad;
                bitmapImage.StreamSource = ms;
                bitmapImage.EndInit();
                mediaItem.Thumbnail = bitmapImage;
                mediaItem.Duration = duration;
                mediaItem.Name = System.IO.Path.GetFileName(name);
                //The protocol we defined expects us to close the MemoryStream here.
                //Otherwise our memory consumption would not be optimized. GC can take its time; so we better do it ourselves!
                ms.Close();
                thumbnailCollection.Add(mediaItem);
                if (--thumbnailsInProgress == 0)
                {
                    downloadAnimation.StopAnimation();
                    animationGrid.Visibility = System.Windows.Visibility.Collapsed;
                }
            }));
        }

        class MediaItem
        {
            public BitmapImage Thumbnail { get; set; }
            public int Duration { get; set; }
            public string Name { get; set; }
        }
    }
}