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

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