Be careful with ChildWindow in the Silverlight SDK. MemoryLeak detected

In the process of searching for my memory leaks in an application written in Silverlight (the last entry in my personal blog) I found an error in the ChildWindow class, leading to a memory leak.

In WinDbg it looks like:

ModalWindow below in my example - the heir from ChildWindow

0: 000: x86>! Dumpheap -mt 05e4f830
Address MT Size
07068c28 05e4f830 236
070ba900 05e4f830 236
total 0 objects
Statistics:
MT Count TotalSize Class Name
05e4f830 2 472 MLOY.MKNA.KarjaKompassi.ModalWindow
Total 2 objects
')
0: 000: x86>! Gcroot 07068c28
Note: Roots found on stacks may be false positives. Run "! Help gcroot" for
more info.
Scan Thread 5 OSTHread 1880
Scan Thread 22 OSTHread 1034
Scan Thread 23 OSTHread 1878
Scan Thread 24 OSTHread 1f28
DOMAIN (062239E0): HANDLE (Pinned): 3cc12f8: Root: 07de4260 (System.Object []) ->
06ed1460 (MLOY.MKNA.KarjaKompassi.FeedingPlan. Shell ) ->
06ed14dc (MS.Internal.CoreTypeEventHelper) ->
06ef0c54 (System.Collections.Generic.Dictionary`2 [[System.Int32, mscorlib], [MS.Internal.CoreTypeEventHelper + EventAndDelegate, System.Windows]]) ->
070ee840 (System.Collections.Generic.Dictionary`2 + Entry [[System.Int32, mscorlib], [MS.Internal.CoreTypeEventHelper + EventAndDelegate, System.Windows]] []) ->
070a68a8 (MS.Internal.CoreTypeEventHelper + EventAndDelegate) ->
070a6870 (System.Windows. RoutedEventHandler ) ->
07068c28 (MLOY.MKNA.KarjaKompassi. ModalWindow )

In the code it looks like (Reflector looked in System.Windows.Controls.dll on the class ChildWindow):

private void ChildWindow_LostFocus (object sender, RoutedEventArgs e)
{
...
Application.Current.RootVisual.GotFocus + = new RoutedEventHandler (this.RootVisual_GotFocus);
...
}

public void Close ()
{
...
Application.Current.RootVisual.GotFocus - = new RoutedEventHandler (this.RootVisual_GotFocus);
...
}

private void SubscribeToEvents ()
{
base.LostFocus + = new RoutedEventHandler (this.ChildWindow_LostFocus);
}

We can subscribe several times (every time the window loses focus), but unsubscribe only once when we call Close ()

Decision:

In the ChildWindow_LostFocus method, replace the problem line with a pair:
Application.Current.RootVisual.GotFocus - = new RoutedEventHandler (this.RootVisual_GotFocus);
Application.Current.RootVisual.GotFocus + = new RoutedEventHandler (this.RootVisual_GotFocus);