hosting clr and catching threading exceptions

Question

I am trying to write an plugin system that can load managed plugins. The host should be able to unload the plugins if there are any exceptions. for my poc I have a sample code library in C# that throws an exception like this ...

 public static int StartUp(string arguments)
 {
       Console.WriteLine("Started exception thrower with args {0}", arguments);
       Thread workerThread = new Thread(() => 
            {
                Console.WriteLine("Starting a thread, doing some important work");
                Thread.Sleep(1000);
                throw new ApplicationException();
            }
         );
         workerThread.Start();
         workerThread.Join();
         Console.WriteLine("this should never print");
        return 11;
    }

then i have native win32 console app like this ..

int _tmain(int argc, _TCHAR* argv[])
{
    ICLRMetaHost *pMetaHost       = NULL;
    HRESULT hr; 
    ICLRRuntimeInfo *runtimeInfo = NULL;    
    __try
    {
        hr = CLRCreateInstance(CLSID_CLRMetaHost, IID_ICLRMetaHost, (LPVOID*)&pMetaHost);
        hr = pMetaHost->GetRuntime(L"v4.0.30319",IID_ICLRRuntimeInfo,(LPVOID*)&runtimeInfo);
        ICLRRuntimeHost *runtimeHost  = NULL;
        hr = runtimeInfo->GetInterface(CLSID_CLRRuntimeHost,IID_ICLRRuntimeHost, (LPVOID*)&runtimeHost);    
        ICLRControl* clrControl = NULL;
        hr = runtimeHost->GetCLRControl(&clrControl);
        ICLRPolicyManager *clrPolicyManager = NULL;
        clrControl->GetCLRManager(IID_ICLRPolicyManager, (LPVOID*)&clrPolicyManager);
        clrPolicyManager->SetDefaultAction(OPR_ThreadAbort,eUnloadAppDomain);   
        hr = runtimeHost->Start();
        DWORD returnVal = NULL;         
        hr = runtimeHost->ExecuteInDefaultAppDomain(L"ExceptionThrower.dll",L"ExceptionThrower.MainExceptionThrower",L"StartUp",L"test",&returnVal);        
        runtimeHost->Release();
    }
    __except(1)
    {
        wprintf(L"\n Error thrown %d",e);
    }
    return 0;
}

Issue is that if i use the above code, the host would complete running the managed code (the line "this should never print" would end up printing) If i remove the clrPolicyManager->SetUnhandledExceptionPolicy(eHostDeterminedPolicy), then the host process would crash.

can anything be done in the unmanaged host that it could gracefully remove the errant app from runtime and continue working ?

Answer 1

First of all, if you want to prevent application crash with the code above, you'll need to use SetUnhandledExceptionFilter, like this:

LONG WINAPI MyUnhandledExceptionFilter(struct _EXCEPTION_POINTERS *exceptionInfo)
{
    // do something useful
    return EXCEPTION_EXECUTE_HANDLER; // prevent crash
}

int _tmain(int argc, _TCHAR* argv[])
{
    SetUnhandledExceptionFilter(MyUnhandledExceptionFilter);
        ...
}

But this may not be what you really want. One solution (as proposed by Polity I believe) is to create an intermediary AppDomain that can catch easily all unhandled exceptions. You can do that in C#, like this:

public class PluginVerifier
{
    public static int CheckPlugin(string arguments)
    {
        AppDomain appDomain = AppDomain.CreateDomain(Guid.NewGuid().ToString());
        appDomain.UnhandledException += AppDomainUnhandledException;
        object obj = appDomain.CreateInstanceAndUnwrap("ExceptionThrower", "ExceptionThrower.MainExceptionThrower");
        object ret = obj.GetType().InvokeMember("Startup", BindingFlags.Instance | BindingFlags.Public | BindingFlags.InvokeMethod, null, obj, new object[] { arguments });
        AppDomain.Unload(appDomain);
        return (int)ret;
    }

    private static void AppDomainUnhandledException(object sender, UnhandledExceptionEventArgs e)
    {
        AppDomain appDomain = (AppDomain)sender;
        // the following will prevent "this should never print" to happen
        AppDomain.Unload(appDomain);
    }
}

For this to be able to work however, you need to do two changes to your plugin classes:

• they must derive from MarshalByRefObject
• the plugin method must not be static (static methods call do not go through AppDomain filter)

So your class would be written like this:

public class MainExceptionThrower: MarshalByRefObject
{
    public int StartUp(string arguments)
    {
    ...
    }
 }

If you do this, you can remove the calls to SetUnhandledExceptionPolicy, SetActionOnFailure, or SetDefaultAction, and just replace the bootstrap code like this:

    hr = runtimeHost->ExecuteInDefaultAppDomain(L"PluginSystem.dll", L"PluginSystem.PluginVerifier", L"CheckPlugin", L"test", &returnVal);        

If you try this with your Startup code above, this call will return hr=0x80131604, which is COR_E_TARGETINVOCATION (TargetInvocationException).

Answer 2

You can start a new AppDomain specifically for each given plugin and launch it inside. See http://msdn.microsoft.com/en-us/library/ms164323.aspx

Each AppDomain is an isolated environment where code can execute. Exceptions occuring in one AppDomain can be isolated from th rest. See: http://msdn.microsoft.com/en-us/library/system.appdomain(v=VS.100).aspx