Global exception handler with UnhandledExceptionEventArgs not working

Question

After reading this MSDN page, I've created a global exception handler in my .net class library, for logging purposes, which looks like this:

    static void OnException(object sender, UnhandledExceptionEventArgs args)
    {
        Exception ex = (Exception)args.ExceptionObject;
        Logging.LogException(ex);
    }

But then if I throw new UnauthorizedAccessException() or throw new Exception() from a method, this does not catch it at all.

The MSDN page says:

UnhandledExceptionEventArgs provides access to the exception object and a flag indicating whether the common language runtime is terminating. The UnhandledExceptionEventArgs is one of the parameters passed into UnhandledExceptionEventHandler for the AppDomain.UnhandledException event

I believe what I'm doing falls under the AppDomain (and not ThreadException)? What am I doing wrong here?

PS. I'm trying to avoid a try-catch block, since apparently it's bad practice. This class library is called from a windows service which runs periodically so I'd rather not let it 'crash' to avoid memory leaks due to unforeseen exceptions and would prefer to monitor the event logs regularly.

This method alone does nothing. Do you subscribe to the UnhandledException event? And if your conclusion from reading the linked Q&A is _"Using try-catch is bad practice"_, you may want to read it again. — CodeCaster, Jan 06 '16 at 13:38
Yes, that's the problem. Although since I don't have a `main` method, where should I be subscribing? As for the attached, it says that in bold. I do agree that it goes into detail but even where the answer talks about logging, it talks about logging and allowing the app to crash. I'm trying to avoid this. — reggaemahn, Jan 06 '16 at 13:42
It says "without a good reason". Anyway you should not want to do this from a class library. It's up to the consuming application how to handle exceptions. — CodeCaster, Jan 06 '16 at 13:49

score 4 · Accepted Answer · answered Jan 06 '16 at 13:59

4

You will need to install the exception handler in the current app domain in order for it to fire:

AppDomain.CurrentDomain.UnhandledException += OnException;

Otherwise its just a method declaration that will never be called.

answered Jan 06 '16 at 13:59

user957902

3,010
14
18

Yes, that is how you subscribe to an event. It is however not a solution to OP's problem, even though they have accepted this answer. The `UnhandledException` event provides you merely with a _notification_ of an unhandled exception. You're not going to be able to _handle_ it there, and you most definitely don't want to do this in **library code** that OP claims to be writing, as you'll be notified of _all_ unhandled exceptions in the appdomain. – CodeCaster Jan 06 '16 at 18:10

score 2 · Answer 2 · answered May 18 '17 at 05:41

You mention that you are trying to avoid a try catch, but inside your handler, that wouldn't be a bad idea:

static void OnException(object sender, UnhandledExceptionEventArgs args)
{
    try
    {
        Exception ex = (Exception)args.ExceptionObject;
        Logging.LogException(ex);
    }
    catch
    {
       // do nothing to silently swallow error, or try something else...
    }
}

...Because you don't want to explode in your error handler. Either swallow if stability is of primary importance, or try a secondary (more basic) logging method to insure that no exception falls through the cracks.

Normally, swallowing an exception silently is a poor practice, but this is inside an error handling block where failure means crashing an app.

Global exception handler with UnhandledExceptionEventArgs not working

2 Answers2