Memory usage is an important aspect, especially on mobile device. If you want to publish an app on the Windows Phone marketplace you must satisfy the Technical Certification Requirements: “5.2.5 Memory Consumption: An application must not exceed 90 MB of RAM usage, except on devices that have more than 256 MB of memory.”

In this post, I’m sharing a technique to track the memory usage of a WP7 app in real tile in every single page of the app. By adding only one line of code in your existing app, you’ll be able to display memory usage in all your pages (without any changes):

Download source code and example

How to integrate the component in your existing app?

1. import the MemoryWatcher class in your existing project
2. in the InitializePhoneApplication method, add a new line after the creation of the RootFrame:

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// Create the frame but don't set it as RootVisual yet; this allows the splash
// screen to remain active until the application is ready to render.
RootFrame = new PhoneApplicationFrame();
// Add the following line !
new MemoryWatcher(RootFrame) { IsDisplayed = true };

How it works?

During its initialization, the MemoryWatcher control will set an event handler to have a callback whenever the user navigates to a new page. When the new page is loaded, it checks if it can dynamically insert the MemoryWatcher control. This is done by checking the root UI element of the page and inserting the watcher control in it. Here is the full code of the MemoryWatcher class:

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using System;
using System.Linq;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Navigation;
using System.Windows.Threading;
 
using Microsoft.Phone.Controls;
using Microsoft.Phone.Info;
 
namespace MemoryWatcherDemo
{
    public class MemoryWatcher : ContentControl
    {
        private readonly DispatcherTimer timer;
        private readonly PhoneApplicationFrame frame;
        private const float ByteToMega = 1024 * 1024;
 
        public bool IsDisplayed { get; set; }
 
        public MemoryWatcher(PhoneApplicationFrame frame)
        {
            if (frame == null)
                throw new ArgumentNullException("frame");
 
            this.frame = frame;
            this.frame.Navigated += new NavigatedEventHandler(this.OnFrameNavigated);
            this.frame.Navigating += new NavigatingCancelEventHandler(this.OnFrameNavigating);
 
            this.timer = new DispatcherTimer { Interval = TimeSpan.FromMilliseconds(100) };
            this.timer.Tick += new EventHandler(this.OnTimerTick);
 
            // setup some basic properties to ensure the content will be visible
            this.Foreground = new SolidColorBrush(Colors.Red);
            this.VerticalContentAlignment = VerticalAlignment.Center;
            this.HorizontalContentAlignment = HorizontalAlignment.Center;
            this.Margin = new Thickness(0, -35, 0, 0);
        }
 
        private void OnFrameNavigated(object sender, NavigationEventArgs e)
        {
            if (!this.IsDisplayed)
                return;
 
            var page = this.frame.Content as PhoneApplicationPage;
            if (page != null)
            {
                Panel host = page.Content as Panel;
                if (host != null && !host.Children.Any(c => c is MemoryWatcher))
                {
                    this.timer.Start();
                    host.Children.Add(this);
                }
            }
        }
 
        private void OnFrameNavigating(object sender, NavigatingCancelEventArgs navigatingCancelEventArgs)
        {
            var page = this.frame.Content as PhoneApplicationPage;
            if (page != null)
            {
                Panel host = page.Content as Panel;
                if (host != null && host.Children.Contains(this))
                {
                    this.timer.Stop();
                    host.Children.Remove(this);
                }
            }
        }
 
        private void OnTimerTick(object sender, EventArgs e)
        {
            try
            {
                string currentMemory = (DeviceStatus.ApplicationCurrentMemoryUsage / ByteToMega).ToString("#.00");
                string peakMemory = (DeviceStatus.ApplicationPeakMemoryUsage / ByteToMega).ToString("#.00");
 
                this.Content = string.Format("Current: {0}MB Peak: {1}MB", currentMemory, peakMemory);
            }
            catch (Exception)
            {
                this.timer.Stop();
            }
        }
    }
}

Note:

• The MemoryWatcher is looking for a Panel type in order to add itself to the list of children in the page. You might want to modify and improve this portion in order to better fit your needs.
• The attached project targets Windows Phone 7.1, if you want to use the code in a 7.0 version, you should change the way the memory values are read (see this article for more details)

Enjoy the code and start tracking memory leaks

Download source code and example

If you want to learn how .Net manages memory in details, understand how the garbage collector works,  what is the next object pointer, how finalizers are managed (and much more !), you should check it out this cool video made by the guys of ANTS Profiler:

source video

• If you’re tied to a XAML file (with a UserControl for instance) you can use an EventSetter
• If you’re using a class that derives from ItemsControl you can override the GetContainerForItemOverride and IsItemItsOwnContainerOverride method to make the control uses your sub class (for example a CustomListBox that uses CustomListBoxItem) and do the job in the sub class (CustomListBoxItem)

Because those 2 approaches weren’t good in our case, we used the EventManager class that is very useful for this kind of operations. In one line of code, you can register an EventHandler for any RoutedEvent of any class:

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public PropertyDataGrid()
{
    EventManager.RegisterClassHandler(
        typeof(DataGridCell), 
        PreviewKeyDownEvent, 
        new KeyEventHandler(this.OnCellPreviewKeyDown));
}

I wrote this code a couple of months ago. Then, a tester explained me that the memory used by our window was never reclaimed. Because the user can open several windows, the memory of the application was growing very quickly.

After spending a couple of hours tracking this memory leak, I found out that it was the call to RegisterClassHandler that was causing the leak. Using a non-static method (OnCellPreviewKeyDown) was causing the EventManager to keep a reference to the PropertyDataGrid control. This strong reference prevented the control for being garbage collected.

The solution was quick and easy; we just make the method static and initialize the handler in the static constructor. This way the delegate does not have a reference to the instance of the control, and can be kept by the EventManager without causing the leak:

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static PropertyDataGrid()
{
    EventManager.RegisterClassHandler(
        typeof(DataGridCell), 
        PreviewKeyDownEvent, 
        new KeyEventHandler(OnCellPreviewKeyDown));
}

And voila ! We move from a solution eating always more memory to something reasonable:

Even using tools such a Memory Profiler, it was difficult to find the source of the leak, but at least, now, I’ll use carefully the EventManager class.

Note that of course you can have memory leak problems too if you don’t unregister your event handlers properly. If you want more information about that you can check out this article on CodeProject.