XAML Binding not reflecting changes in ViewModel

Question

I have the following code:

MoviePanel.xaml

<StackPanel Orientation="Horizontal" >
        <Border BorderThickness="3" BorderBrush="Black">
            <DockPanel Width="350">
                <StackPanel  VerticalAlignment="Top" Height="200">
                    <TextBlock Width="200" Height="50" DockPanel.Dock="Top" HorizontalAlignment="Left">
                        <TextBlock.Text>
                            <MultiBinding StringFormat="{}{0} ({1})">
                                <Binding Path = "_selectedMovie.title"/>
                                <Binding Path = "_selectedMovie.year"/>
                            </MultiBinding>
                        </TextBlock.Text>
                    </TextBlock>
                </StackPanel>
                <StackPanel VerticalAlignment="Top" Height="200">
                    <Grid HorizontalAlignment="Center">
                        <Image Source="star_icon.png" Width="100"/>
                        <TextBlock Text="{Binding _selectedMovie.rating}" Style="{StaticResource AnnotationStyle}" Width="150"/>
                    </Grid>
                </StackPanel>
            </DockPanel>
        </Border>
</StackPanel>

MoviePanel.xaml.cs

public MoviePanel()
    {
        InitializeComponent();
        _moviePanelVM = new MoviePanelViewModel();
        _moviePanelVM.GetAllMovieDetails();            
        this.DataContext = _moviePanelVM;            
    }

MoviePanelViewModel.cs

 public class MoviePanelViewModel : INotifyPropertyChanged
 {
    // List to hold movieDetail objects 
    public List<MediaDetail> _movies { get; set; }

    public MediaDetail _selectedMovie { get; set; }

    public string selectedMovieID { get; set; }

    GetSelectedMovieDetails()
    {
       // Set _selectedMovie here
    }

MediaDetail

public class MediaDetail : DependencyObject {

    public ImageSource image {get; set;}        
    public string title {get; set;}
    public string year { get; set; }
    public string id { get; set; }
    public string rating { get; set; }
    public string description { get; set; }
    public string[] cast{ get; set; }
    public string[] director { get; set; }
    public string[] genre { get; set; }
    public string votecount { get; set; }
    public string[] language { get; set; }
    public string runtime { get; set; }
    public string part { get; set; }
    public string tagline { get; set; }

    public bool isMovieSelected
    {
        get { return (bool)this.GetValue(isMovieSelectedProperty); }
        set
        {
            this.SetValue(isMovieSelectedProperty, value);
        }
    }
    public static readonly DependencyProperty isMovieSelectedProperty = DependencyProperty.Register(
                                                             "isMovieSelected", typeof(bool),
                                                             typeof(MediaDetail));

}

The problem I am having is that my UI is not being updated. That is _selectedMovie.title, _selectedMovie.year and _selectedMovie.rating are not being updated.

In another part of my XAML, I bind to _movies and this seems to work fine.

Would anyone have any ideas as to why this might be the case?

Thank you for your help.

UPDATE:

Based on the accepted answer below I changed:

MoviePanelViewModel.cs

 public class MoviePanelViewModel : INotifyPropertyChanged
 {
      private MediaDetail _selectedMovie;
    public MediaDetail SelectedMovie
    {
        get { return _selectedMovie; }
        set
        {
            _selectedMovie = value;
            OnPropertyChanged("SelectedMovie");
        }
    }
 }

Answer 1

None of that actually is using INotifyPropertyChanged (ie, raising the PropertyChanged event), so the UI can't receive the updates.

In particular, MediaDetail doesn't even derive from INotifyPropertyChanged, so when you bind to its properties, there is no way it will pick up the updates:

<!-- Binds to properties of MediaDetail! -->
<Binding Path = "_selectedMovie.title"/>
<Binding Path = "_selectedMovie.year"/>

For any property whose changes need to propagate to the UI, make sure it raises PropertyChanged and the UI will update as expected.