Alois Kraus


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Recently I had an interesting issue to diagnose. We all (readers of my blog) know that forgotten unregister calls to events are a common source of managed memory leaks. The weak event pattern is a common cure to this plague and WPF has a nice implementation for it. With .NET 4.5 WeakEventManager has got a new cousin which does not require you to derive from it which is even nicer. Every time when I needed to deal with weak event registrations I used it and it worked fine. Since WeakEventManager is a low level class I have used it also in non ui applications which turned out to be a bad design decision.

What do you think will this fine sample application do?

    class Program
        public static event EventHandler OnSomething;

        static void Main(string[] args)
            var p = new Program();
            for(int i=0;i<1000*1000;i++)
                ThreadStart func = () =>
                        // Register a weak event handler 
                        WeakEventManager<Program,EventArgs>.AddHandler(p, "OnSomething", (o,e)=> {});

                // Do the registration every time on a new thread
                var thread = new Thread(func);

            // lets see how far we get

No not the answer ….

Still no

It will give you this strange looking exception:

System.ComponentModel.Win32Exception: Not enough storage is available to process this command

   at MS.Win32.UnsafeNativeMethods.RegisterClassEx(WNDCLASSEX_D wc_d)
   at MS.Win32.HwndWrapper..ctor(Int32 classStyle, Int32 style, Int32 exStyle, Int32 x, Int32 y, Int32 width, Int32 height, String name, IntPtr parent, HwndWrapperHook[] hooks)
   at System.Windows.Threading.Dispatcher..ctor()
   at System.Windows.Threading.Dispatcher.get_CurrentDispatcher()
   at MS.Internal.WeakEventTable..ctor()
   at MS.Internal.WeakEventTable.get_CurrentWeakEventTable()
   at System.Windows.WeakEventManager`2.CurrentManager(String eventName)
   at System.Windows.WeakEventManager`2.AddHandler(TEventSource source, String eventName, EventHandler`1 handler)
   at WhatsWrongWithThisCode.Program.<>c__DisplayClass3.<Main>b__0() in d:\Media\Blog\WhatsWrongWithThisCode\WhatsWrongWithThisCode\Program.cs:Zeile 23.
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()


What happened here? The WeakEventManager has a hidden dependency to a Dispatcher object which assumes that you live on a thread that has an active window message pump. My newly created threads did never pump messages which causes the scheduled background operations to cleanup garbage collected event subscribers to hang forever. If you execute this logic on only a few threads nothing bad will happen. But if you use this in a server application which runs for weeks on dynamically generated threads then your task manager will look like this:




All looks normal except that we seem to leak many handles. In fact we do leak thread handles. It seems that the RegisterClassEx causes the managed thread objects to get pinned. That is not nice but no big deal. Windows can handle many more handles. But here is the catch:

  • You just have screwed your whole machine!

All of a sudden you cannot open new applications. Windows cannot be created, the C++ compiler will stop with an error that no .NET Framework is installed and many other very odd messages from any application can happen. At this point you usually reboot the server and hope that the error does not show up too often. Did you let this application run? Congratulations. A reboot is required Zwinkerndes Smiley quite soon.

When you google for the error message you get some links back that a non paged pool allocation failed and you fix it by changing some magic registry keys. That's what I did but it did not help. My application did not run out of non paged pool memory but it ran out of Desktop Heap. If you have never heard about Desktop Heap you are in good company. This heap is global for all applications in your session and many Window related methods allocate stuff from it. According to the NT debugging blog it is mainly used by CreateWindow but other functions (which are not documented) can also cause Desktop Heap allocations. Apparently it seems that we have just found another method named RegisterClassEx which also allocates from the Desktop Heap. You can change the desktop heap size for your interactive session by changing the registry key

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\SubSystems\Windows

which has on my Windows 8.1 box this value

%SystemRoot%\system32\csrss.exe ObjectDirectory=\Windows SharedSection=1024,20480,768 Windows=On SubSystemType=Windows ServerDll=basesrv,1 ServerDll=winsrv:UserServerDllInitialization,3 ServerDll=sxssrv,4 ProfileControl=Off MaxRequestThreads=16

The yellow marked value is the one you usually care about which is the size of your interactive desktop heap in KB. 20 MB is not so much but under Windows XP it was only 3MB where this error was much more common. By increasing this value to an even bigger one I was able to make this quite often happening issue to a very sporadic one which made this a not urgent issue. I tried to analyze the desktop heap usage of a kernel dump with !dheap but this Windbg command did never work so I was stuck until I did check the handle leak. When I have tracked down the root cause of the handle leak to the WPF WeakEventManager class it all made sense. The bad thing was that I always searched for high values of User Objects (window handles) which never happened when the machine ran out of desktop heap, leaving not many clues which process was responsible for allocating so much desktop heap.

What is the morale of this story? Never use WeakEventManager in threads which have no Window message pump running or you will run out of desktop heap sooner or later if you call it from many threads!

Reason found. Now what? When you search for Weak event patterns on the net you will find samples which

Although the concept is simple, a working implementation which works in all circumstances seems to be surprisingly hard to create. So far I have found FastSmartWeakEvent from Daniel Grunwald the best implementation which keeps the original code with += and -= to register and unregister a weak delegate and does even throw an exception at you when you try to register a delegate which contains closures which would cause the event never to fire because the compiler generated closure class is not referenced by anyone except the weak (too weak) delegate which is collected as soon as the method where the registration happened is left. Next time you see "Not enough storage is available to process this command" you know now where to look at. If the handle count is high the odds are good that someone did use the WPF WeakEventManager on non ui threads like I did.

posted on Tuesday, February 4, 2014 10:05 AM