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.NET AppDomain.Unload triggers runaway threads

I have an .NET 3.5 SP1 application that is an Excel Add-in. The application is split into a parent AppDomain (Excel's) and a child domain in which we load all of our dlls. When we wish to update our application, we unload the child domain, replace the files and reload it.

Unfortunately unloading the domain will activate 2 worker threads and they will start consuming CPU cycles (20-40%).

If I debug with VS 2010, at the moment before and after the AppDomain.Unload, there are no threads alive with a call stack aside from Excel's main thread. The AppDomain.Unload is indeed unloaded because if I try to call Unload again I get an AppDomainUnloadedException.

If I use ProcessExplorer, I can see that 2 threads are busy working away, even when the VS debugger has breaked. Looking at the callstack reveals nothing because there are no symbols.

ntkrnlpa.exe+0x6eacb
ntkrnlpa.exe+0x2bfd0
hal.dll+0x2ef2
ntkrnlpa.exe+0x6a6cf
ntdll.dll+0xe514
mscorwks.dll+0x992d
mscorwks.dll+0x52568
mscorwks.dll+0x15b469
kernel32.dll+0xb729

If I use WinDbg, I can see the callstack for the 2 renegade threads. It's always the same thing:

WARNING: Stack unwind information not available. Following frames may be wrong.
ntdll!KiFastSystemCallRet
mscorwks+0x992d
mscorwks!InstallCustomModule+0x1eca0
mscorwks!CorExitProcess+0x503b
kernel32!GetModuleFileNameA+0x1ba

I created a very simple test application to load/unload a child assembly. When doing this with a simple 1-id assembly, it works without any problems. If I get it to load/unload the child domain of the real application, it triggers the same renegade threads.

The code that creates the child domain is as follows:

AppDomainSetup appSetup = new AppDomainSetup();
appSetup.ApplicationBase = baseDir;

var ps = new PermissionSet(System.Security.Permissions.PermissionState.Unrestricted);
return AppDomain.CreateDomain(name, null, appSetup, ps, null);

The communication from the parent to the child domain is via a proxy and reflection. The code to create it is below:

string assName = typeof(ApplicationProxy).Assembly.FullName;
string idName = typeof(ApplicationProxy).FullName;

var obj = _childDomain.CreateInstanceAndUnwrap(assName, idName, false,
System.Reflection.BindingFlags.Default,
null, new object[]{_sessionGuid},
CultureInfo.InvariantCulture,
null, new Evidence(AppDomain.CurrentDomain.Evidence));

_proxy = (ApplicationProxy)obj;

I've googled the problem profusely and cannot find anybody with a similar problem. The application is 10 projects large so I can't post it.

I'm wondering if anybody has encountered something similar and has some tips for me. Otherwise does anybody have any thoughts on how to attack the problem?

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Forum posts for ntkrnlpa.exe

.NET AppDomain.Unload triggers runaway threads

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What is the carbon footprint of your coffee?