C# not catching unhandled exceptions from unmanaged C++ dll

Question

I've got an unmanaged C++ dll which is being called from a C# app, I'm trying to get the C# app to catch all exceptions so that in the event of the dll failing due to an unmanaged exception then the user will get a half-decent error message (the C# app is a web service implementing it's own http handler).

The problem I have is that not all types are being caught. So if I create the following and execute the C# app then the dll throws an error and the entire application terminates. Any ideas?

This is being created in VS2005 and using .Net framework v2

C++ - Test.h

#ifndef INC_TEST_H
#define INC_TEST_H

extern "C" __declspec(dllexport) void ProcessBadCall();

#endif

C++ - Test.cpp

#include <iostream>
#include <vector>

using namespace std;

void ProcessBadCall()
{
  vector<int> myValues;
  int a = myValues[1];
  cout << a << endl;
}

C# - Program.cs

class Program
{
  [DllImport("Test.dll", EntryPoint="ProcessBadCall")]
  static extern void ProcessBadCall();

  static void Main(string[] args)
  {
    try
    {
      ProcessBadCall();
    }
    catch (SEHException ex)
    {
      Console.WriteLine("SEH Exception: {0}", ex.Message);
    }
    catch (Exception ex)
    {
      Console.WriteLine("Exception: {0}", ex.Message);
    }
  }
}

The dll is being compiled under the release configuration with the following compiler flags.

/O2 /GL /D "WIN32" /D "NDEBUG" /D "_CRT_SECURE_NO_WARNINGS" /D "_UNICODE" /D "UNICODE" /D "_WINDLL" /FD /EHa /MD /Fo"Release\" /Fd"Release\vc80.pdb" /W4 /WX /nologo /c /Wp64 /Zi /TP /errorReport:prompt

when the dll is compiled in debug mode, this wont raise a SEH Exception, but will cause an assertation failure. — smerlin
Sorry, missed that bit out, the dll is compiled in release mode — daz-fuller
Just tried that, still no luck. What seems to be happening is that the DLL is throwing an unhandled exception which terminates both itself and the c# app immediately — daz-fuller
You should catch the exception in the same runtime that threw it. Else stack unrolling and destructor calls might not work as expected. And you typicall can't recover from a access violation, so you should terminate the application after displaying the error. — CodesInChaos

Zach Saw Zach Saw · Accepted Answer · 2010-11-19T11:28:30

Try catching using the ExternalException class:

http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.externalexception%28v=VS.100%29.aspx

And, try compiling your unmanaged C++ DLL with asynchronous exception handling (/EHa). It looks like you're getting a Read Access Violation in your DLL which is a type of async exception.

AFAIK, only .NET v4 and above disables the delivery of async exceptions by default. Even then, you could add legacyCorruptedStateExceptionsPolicy=true to the app.config to enable it. Prior to that, it's automatically enabled (check that you have got it set to false in app.config).

Note that it's my personal believe that AVs in your unmanaged DLL is inherently bad (and dangerous) anyway and it's probably the right behavior for .NET to simply terminate the app. Try throwing std::exception instead. If you insists on catching async exceptions, the best way would be to have a thunking DLL which wraps try-catch-all around the call to potentially buggy function calls. Again, highly /not/ recommended (although I can see how it would be useful in debugging the misbehaving DLL).

C# not catching unhandled exceptions from unmanaged C++ dll

1 Answers