c# - Garbage collection fails to reclaim BitmapImage?

Question

I have an application(WPF) which creates BitmapImages in huge numbers(like 25000). Seems like framework uses some internal logic so after creation there are approx 300 mb of memory consumed(150 virtual and 150 physical). These BitmapImages are added into Image object and they are added into Canvas. The problem is that when I release all those images memory isn't freed. How can I free memory back?

The application is simple: Xaml

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Canvas x:Name="canvas" Grid.ColumnSpan="2"></Canvas>
        <Button Content="Add" Grid.Row="1" Click="Button_Click"/>
        <Button Content="Remove" Grid.Row="1" Grid.Column="1" Click="Remove_click"/>
    </Grid>

Code-behind

        const int size = 25000;
        BitmapImage[] bimages = new BitmapImage[size];
        private void Button_Click(object sender, RoutedEventArgs e)
        {
            var paths = Directory.GetFiles(@"C:Images", "*.jpg");
            for (int i = 0; i < size; i++)
            {
                bimages[i] = new BitmapImage(new Uri(paths[i % paths.Length]));
                var image = new Image();
                image.Source = bimages[i];
                canvas.Children.Add(image);
                Canvas.SetLeft(image, i*10);
                Canvas.SetTop(image, i * 10);
            }
        }

        private void Remove_click(object sender, RoutedEventArgs e)
        {
            for (int i = 0; i < size; i++)
            {
                bimages[i] = null;
            }
            canvas.Children.Clear();
            bimages = null;
            GC.Collect();
            GC.Collect();
            GC.Collect();
        }

This is a screenshot of ResourceManager after adding images

Answer 1

There was a bug in Wpf that we were bitten by where BitmapImage objects are not released unless you freeze them. https://www.jawahar.tech/home/finding-memory-leaks-in-wpf-based-applications was the original page where we discovered the issue. It should have been fixed in Wpf 3.5 sp1 but we were still seeing it in some situations. Try changing your code like this to see if that is the problem:

bimages[i] = new BitmapImage(new Uri(paths[i % paths.Length]));
bimages[i].Freeze();

We routinely freeze our BitmapImage objects now as we were seeing other instances in the profiler where Wpf was listening for events on the BitmapImage and thereby keeping the image alive.

If the Feeze() call isn't an obvious fix for your code, I would highly recommend using a profiler such as the RedGate Memory Profiler - that will trace a dependency tree that will show you what it is that is keeping your Image objects in memory.