What is caching the image data in my Silverlight 4 application?

Question

Something is holding on to the images in my Silverlight application. This is a problem because this is a catalog application with a lot of images that the user can browse through. If the user browses through every library provided the application will run out of memory and crash. I know it is a problem with images because if I disable images the private working set never exceeds 170 MB. I have a user control (DownloadImage) that shows a progress bar while the image is downloading. It is the only thing in my code that is referencing the BitmapImage objects. I ran a profiler (ANTS Memory Profiler 7.0 -- good product, just giving them props) and it showed that all of the BitmapImage instances are being properly collected. As are all instances of DownloadImage controls.

Here is the code behind of DownloadImage:

    public partial class DownloadImage : UserControl
    {
        public static readonly DependencyProperty SourceProperty = DependencyProperty.RegisterAttached("Source", typeof(Uri), typeof(DownloadImage), new PropertyMetadata(null, OnSourcePropertyChanged));

        /// <summary>
        /// Default C'tor
        /// </summary>
        public DownloadImage()
        {
            InitializeComponent();
        }

        /// <summary>
        /// D'tor
        /// </summary>
        ~DownloadImage()
        {
        }

        /// <summary>
        /// Source - Image source.
        /// </summary>
        public Uri Source
        {
            get { return (Uri)GetValue(SourceProperty); }
            set { SetValue(SourceProperty, value); }
        }

        /// <summary>
        /// Initialize this DownloadImage with the given <see cref="BitmapImage"/> (<paramref name="a_bitmapImage"/>).
        /// </summary>
        /// <param name="a_bitmapImage">Given <see cref="BitmapImage"/>.</param>
        public void Create(BitmapImage a_bitmapImage)
        {
            _noImage.Visibility = Visibility.Collapsed;

            _image.Source = a_bitmapImage;
        }

        /// <summary>
        /// Initialize this DownloadImage with the given image source <see cref="Uri"/> (<paramref name="a_imageSource"/>).
        /// </summary>
        /// <param name="a_imageSource">Given image source <see cref="Uri"/>.</param>
        public void Create(Uri a_imageSource)
        {
            _noImage.Visibility = Visibility.Collapsed;

            BitmapImage bitmapImage = new BitmapImage();
            bitmapImage.CreateOptions = BitmapCreateOptions.DelayCreation | BitmapCreateOptions.IgnoreImageCache;
            bitmapImage.DownloadProgress += new EventHandler<DownloadProgressEventArgs>(OnDownloadProgress);
            bitmapImage.ImageOpened += new EventHandler<RoutedEventArgs>(OnDownloadSuccess);
            bitmapImage.ImageFailed += new EventHandler<ExceptionRoutedEventArgs>(OnDownloadFailed);
            bitmapImage.UriSource = a_imageSource;

            _image.Source = bitmapImage;
        }

        /// <summary>
        /// When the download progress changes.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        protected virtual void OnDownloadProgress(object sender, DownloadProgressEventArgs e)
        {
            _progress.Visibility = Visibility.Visible;
            _progress.Value = e.Progress;
        }

        /// <summary>
        /// When the download succeeds.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        protected virtual void OnDownloadSuccess(object sender, RoutedEventArgs e)
        {
            _noImage.Visibility = Visibility.Collapsed;
            _progress.Visibility = Visibility.Collapsed;
        }

        /// <summary>
        /// When the download fails.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        protected virtual void OnDownloadFailed(object sender, ExceptionRoutedEventArgs e)
        {
            _noImage.Visibility = Visibility.Visible;
            _progress.Visibility = Visibility.Collapsed;
        }

        /// <summary>
        /// When SourceProperty dependency property changes.
        /// </summary>
        /// <param name="obj"></param>
        /// <param name="e"></param>
        private static void OnSourcePropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            DownloadImage downloadImage = obj as DownloadImage;
            if (downloadImage != null)
            {
                if (e.NewValue is Uri)
                    downloadImage.Create(e.NewValue as Uri);
                else if (e.NewValue is BitmapImage)
                    downloadImage.Create(e.NewValue as BitmapImage);
                else
                    return;
            }
        }

    }

And here is the XAML:

<UserControl x:Name="userControl" x:Class="{Intentionally Left Blank}.DownloadImage"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:helpers="clr-namespace:{Intentionally Left Blank}.Helpers"
             xmlns:res="clr-namespace:{Intentionally Left Blank}.Resources"
             xmlns:ee="http://schemas.microsoft.com/expression/2010/effects"
             mc:Ignorable="d"
             d:DesignHeight="100" d:DesignWidth="100" Background="{StaticResource ImageBackground}">
    <Grid x:Name="LayoutRoot" Background="{Binding Background, ElementName=userControl}">
        <Viewbox HorizontalAlignment="Center" VerticalAlignment="Center" Margin="8">
            <TextBlock x:Name="_noImage" TextWrapping="Wrap" FontSize="16"
                res:Strings.Assignment="Text=DownloadImage.NoImage" Height="23" Width="79" Visibility="Collapsed">
                <TextBlock.Foreground>
                    <SolidColorBrush Color="{StaticResource GrayLetters}"/>
                </TextBlock.Foreground>
            </TextBlock>
        </Viewbox>
        <Image x:Name="_image"/>
        <ProgressBar x:Name="_progress" Height="15" VerticalAlignment="Bottom" Margin="1" Visibility="Collapsed"/>
    </Grid>
</UserControl>

Answer 1

I'm not sure why it worked, but I added the following code to DownloadImage and memory is stable.

    /// <summary>
    /// Default C'tor
    /// </summary>
    public DownloadImage()
    {
        InitializeComponent();

        Unloaded += new RoutedEventHandler(OnUnloaded);
    }

    /// <summary>
    /// When the control is unloaded.
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void OnUnloaded(object sender, RoutedEventArgs e)
    {
        BitmapImage bitmapImage = _image.Source as BitmapImage;
        if (bitmapImage != null)
        {
            bitmapImage.DownloadProgress -= new EventHandler<DownloadProgressEventArgs>(OnDownloadProgress);
            bitmapImage.ImageOpened -= new EventHandler<RoutedEventArgs>(OnDownloadSuccess);
            bitmapImage.ImageFailed -= new EventHandler<ExceptionRoutedEventArgs>(OnDownloadFailed);
        }

        _image.Source = null;
    }