是否有任何事件在发生异常时被触发？

Question

Is there any event in .Net framework which gets fired on exception. Whenever there is an exception is caught, i need to log it. So if there an event exist, i can subscribe to that and can log the exceptio in the event handler.

Answer 1

Yes - there is the UnhandledException event on the AppDomain object:

AppDomain.CurrentDomain.UnhandledException += YourHandler

FYI: You should only use these handlers as a last resort - it is far better to catch exceptions in a try catch block, although this may not always be possible (for example in the case when 3rd party code starts new threads)

Also, this event will only be fired when an exception is unhandled - to my knowledge there is no way of being notified of caught events in this way without attaching a debugger to the process.

Answer 2

You should have a look at doing some Aspect Oriented Programming with PostSharp, you can write a simple Log attribute and apply it to your whole assembly.

All you need is something like this:

[Serializable]
public class LogAttribute : OnMethodInvocationAspect
{
   public override void OnInvocation(MethodInvocationEventArgs eventArgs)
   {
      try
      {
         eventArgs.Proceed();
      }
      catch(Exception ex)
      {
         // log exception here
      }
   }
}

and apply it to your assembly:

[assembly: Log]
public class ...

It's not everyone's cup of tea, but I've found it a very clean, neat way to avoid doing a lot of boilerplate code in my classes and frees me up to work on functionalities more related to the project itself.

Update: as Kugel pointed out in the comment, this will help you track and log any exceptions thrown during the execution of the method, but if you want to log the state internal to the method you'll need to do a little more work than this.

For instance, you might still need try/catch blocks inside your method which you could use to capture exceptions that are of interest to your class and maybe even wrap them in a custom exception object so you can start adding more useful information like an error code, etc. So long your custom exception has a suitable mechanism of setting its 'Message' property, eg

public class DictionaryKeyNotValidException() : Exception
{
   public DictionaryKeyNotValidException(string key)
       : base(GetMessage(key))
   {   
   }

   public ErrorEnum ErrorCode { get { return ErrorEnum.InvalidDictionaryKey; } }

   private string GetMessage(string key)
   {
      return string.Format("ERROR {0} : Invalid dictionary key encountered {1}",
                           ErrorCode.GetHashCode(), key);
   }
}

then in your log attribute, provided you're using Log4Net, you can start logging more useful information:

catch (Exception ex)
{
   // log error
   log.Error(ex);

   // handle exception, rethrow, etc.
   ...
}

Sorry this is becoming a bit long winded..

Answer 3

Besides the Exception ? I'd just add a call to my logging module into my catch block.

Something like:

catch(YourException ex)
{
  LogMyException(ex, [otherParamsYouNeed]);
  //Other Exception Handling
}

If you want to log the status of something regardless of success or failure, use finally{}

Answer 4

In WinForms you have the Application.ThreadException event and more general there is the AppDomain.CurrentDomain.UnhandledException event.

But please note that after logging these exceptions the advised thing to do is close the application. It may no longer be in a stable state.

Answer 5

AppDomain类有一个UnhandledException事件，但我认为你不能订阅任何抛出异常。

Answer 6

look at this http://www.switchonthecode.com/tutorials/csharp-tutorial-dealing-with-unhandled-exceptions UnhandledExceptionEventHandler is what you are looking for this is with windows from but i think there is similar think for the web too
Best Regards,
Iordan

Answer 7

In ASP.NET, unhandled exceptions can be caught at the page level by handling the Page_Error event, or at the application level (in global.asax, or in an IHttpModule implementation) by handling Application Error events. These events don't give you the actual exception, so you'll need to call the server to get the exception:

    Exception ex = HttpContext.Current.Server.GetLastError();

Answer 8

To extend on the answer given by theburningmonk, you could also try using log4PostSharp . This is pre-built code that uses PostSharp to insert logging code that works with the log4net logging framework.

Amongst other things this will automatically add code to your methods that catches exceptions and logs them.

It all depends on whether you are happy to use the log4net logging framework or would prefer to do it yourself.

Answer 9

In vb.net, one can add a "When" qualifier to a catch statement (eg 'Catch Ex as ObjectDisposedException When Ex.Message.Contains("Sorry")'). This may be used for logging putposes (eg 'Catch Ex as Exception When LoggingFunctionThatReturnsFalse(Ex)' won't catch any exceptions, but will log them all) and seems like a feature that would have been useful in C#. Last I heard, though, the feature was only available in vb.net (not sure what would happen if one compiled code using such a feature in vb.net and decompiled to C#).

Answer 10

You could put something in the Global.asax Application_Error event.

I've used this method to do exactly what you described in the original post, logging the error and emailing a notification.