DispatcherTimer memory leak in Silverlight 4

A colleague of mine found what seems to be a bug in Silverlight 4. Once started, DispatcherTimer object is never garbage collected. The following code snippet demonstrates the issue:

public partial class MainPage : UserControl {

    public MainPage() {

        InitializeComponent();

    }

    private void start_Click(object sender, RoutedEventArgs e) {

        DispatcherTimer timer = new DispatcherTimer();

        timer.Interval = TimeSpan.FromSeconds(1);

        timer.Tick += timer_Tick;

        timer.Start();

    }

    void timer_Tick(object sender, EventArgs e) {

        DispatcherTimer timer = (DispatcherTimer)sender;

        timer.Stop();

        timer.Tick -= timer_Tick;

    }

}

 

Start application, attach WinDbg. Initially there is no DispatcherTimer object instances:

0:028> !dumpheap -type DispatcherTimer
Address       MT     Size
total 0 objects
Statistics:
      MT    Count    TotalSize Class Name
Total 0 objects

Then click “Start” button. Wait a couple of seconds – the timer object fires, stops and unsubscribes from event handler. Now break into the debugger once again:

0:003> !dumpheap -type DispatcherTimer
Address       MT     Size
08ff5df8 629c4ba0       20    
total 0 objects
Statistics:
      MT    Count    TotalSize Class Name
629c4ba0        1           20 System.Windows.Threading.DispatcherTimer
Total 1 objects
0:003> !gcroot -nostacks 08ff5df8
DOMAIN(07F1AE30):HANDLE(Pinned):43b12f8:Root:  09f74260(System.Object[])->
  08fa5f64(System.Collections.Generic.Dictionary`2[[System.IntPtr, mscorlib],[System.Object, mscorlib]])->
  08feafc4(System.Collections.Generic.Dictionary`2+Entry[[System.IntPtr, mscorlib],[System.Object, mscorlib]][])->
  08ff5df8(System.Windows.Threading.DispatcherTimer)

As you can see, the timer object is “rooted” in Dictionary entry and it will prevent it from being garbage collected.