Why not catch general Exceptions

问题:

My VS just told me;

Warning 2 CA1031 : Microsoft.Design : Modify 'Program.Main(string[])' to catch a more specific exception than 'Exception' or rethrow the exception.

Why should I do that? If I do so, and don't catch all exceptions to handle them, my program crashes with the all-popular report-screen. I don't want my users to get such error-crap!

Why should I not catch all exceptions at once to display a nice warning to the user saying: "Something went wrong, don't care about it, I will handle it, just be patient"?

Edit: Just saw I have a dupe here, sorry for that Dupe

Edit2: To clarify things; I do exit the program after any exception has been catched! I just don't want my user to see that "report to microsoft" dialog that show up when an unhandled exception is raised in a console-application!

回答1:

Swallowing exceptions is a dangerous practice because:

• It can cause the user to think something succeeded when it actually failed.
• It can put your application into states that you didn't plan for.
• It complicates debugging, since it's much harder to find out where the failure happened when you're dealing with bizarre/broken behavior instead of a stack trace.

As you can probably imagine, some of these outcomes can be extremely catastrophic, so doing this right is an important habbit.

Best Practice

First off, code defensively so that exceptions don't occur any more than necessary. They're computationally expensive.

Handle the expected exceptions at a granular level (for example: FileNotFoundException) when possible.

For unexpected exceptions, you can do one of two things:

• Let them bubble up normally and cause a crash
• Catch them and fail gracefully

Fail Gracefully?

Let's say you're working in ASP.Net and you don't want to show the yellow screen of death to your users, but you also don't want problems to be hidden from the dev team.

In our applications, we usually catch unhandled exceptions in global.asax and then do logging and send out notification emails. We also show a more friendly error page, which can be configured in web.config using the customErrors tag.

That's our last line of defense, and if we end up getting an email we jump on it right away.

That type of pattern is not the same as just swallowing exceptions, where you have an empty Catch block that only exists to "pretend" that the exception did not occur.

Other Notes

In VS2010, there's something called intellitrace coming that will allow you to actually email the application state back home and step through code, examine variable values at the time of the exception, and so on. That's going to be extremely useful.

回答2:

Because programs that swallow (catch) exceptions indiscriminately, (and then continue), cannot be relied upon to do what it is they are expected to do. This is because you have no idea what kind of exception was "ignored". What if there was an overflow or memory access error that causes the wrong amount to be debited from a financial account? What if it steers the ship into the iceberg instead of away from it ? Unexpected failures should always cause the application to terminate. That forces the development process to identify and correct the exceptions it finds, (crashes during demos are a wonderful motivator), and, in production, allows appropriately designed backup systems to react when the software experiences an "unexpected" inability to do what it was designed to do.

EDIT: To clarify distinctions between UI components, and service or middleware componentrs.

In Service or Middleware components, where there is no user interacting with the code component from within the same process space that the code is running in, the component needs to "pass On" the exception to whatever client component imnitiated the call it is currently processing. No matter the exception, it should make every possible attempt to do this. It is still the case, however, tjhat in cases where an unexpected, or unanticipated exception occurs, the component should finally terminate the process it is running in. For anticipated or expected exceptions, a velopment analysis should be done to determine whether or not, for that specific exception, the component and it's host process can continue to operate (handling future requests), or whether it should be terminated.

回答3:

You should handle the exact exceptions you are capable of handling and let all others bubble up. If it displays a message to the user that means you don't quite know what you can handle.

回答4:

Having worked on equipment used by emergency responders, I would rather the user see an ugly error message than to accidently swallow an exception that misleads the user into believing everything is "ok". Depending on your application, the consequence could be anything from nothing to a lost sale to a catastrophic loss of life.

If a person were going to catch all exception, show a better error dialog, and then quit the application, that's ok.. but if they are going to continue running after swallowing an unknown exception, I would fire a person for that. It's not ok. Ever.

Good coding is about practices that assume humans make mistakes. Assuming all "critical" exceptions have been caught and handled is a bad idea.

回答5:

Simple answer: you are supposed to fix your bug. Find the place that throws the exception and unless it is beyond your control - fix it. Also catching (without rethrowing) all kinds of exception violates exception neutrality. In general you do not want to do this (although catching exceptions in main does look like special case)

回答6:

Since your warning message shows that this is in Main(), I'll assume that in lower levels, you do catch only more specific Exceptions.

For Main(), I'd consider two cases:

1. Your own (debugging) build, where you want all the exception information you can get: Do not catch any Exceptions here, so the debugger breaks and you have your call stack,
2. Your public releases, where you want the application to behave normally: Catch Exception and display a nice message. This is always better (for the average user) than the 'send report' window.

To do this nicely, just check if DEBUG is defined (and define it, if VS doesn't do this automatically):

#if DEBUG
  yadda(); // Check only specific Exception types here
#else
  try
  {
    yadda();
  }
  catch (Exception e)
  {
     ShowMessage(e); // Show friendly message to user
  }
#endif

I'd disable the warning about catching general Exceptions, but only for your Main() function, catching Exception in any other method is unwise, as other posters have said already.

回答7:

There is a way to suppress certain messages from code analysis. I've used this for this exact reason (catching the general exception for logging purposes) and it's been pretty handy. When you add this attribute, it shows you've at least acknowledged that you are breaking the rule for a specific reason. You also still get your warning for catch blocks that are incorrect (catching the general exception for purposes other than logging).

MSDN SuppressMessageAttribute

回答8:

When you catch general exceptions, you get the side effect of potentially hiding run-time problems from the user which, in turn, can complicate debugging. Also, by catching general exception, you're ignoring a problem (which you're probably throwing elsewhere).

回答9:

You can set up your try catch to catch multiple different behavior types and handle the exception based on the type. For most methods and properties in the framework, you can also see what exceptions they are capable of throwing. So unless you are catching an exception from an extremely small block of code, you should probably catch specific exceptions.

回答10:

In VS you can setup a custom error page to show your users when something goes wrong instead of catching it in a try-catch. I'm assuming since you're using VS that you're using ASP .NET. If so add this tag to your Web.Config under the System.Web tag:

<customErrors mode="RemoteOnly" defaultRedirect="~/CustomErrorPage.aspx" redirectMode="ResponseRewrite" />

You can also catch all uncaught exceptions in the Global.asax file (if you don't have it already: Right-click on web project, select Add Item, and search for it). There are a bunch of application wide event handlers in that file like "Application_Error" that catches every exception that isn't caught within your application so you don't have to use Try-Catch all the time. This is good to use to send yourself an email if an exception occurs and possibly redirect them to your homepage or something if you don't want to use the customErrors tag above.

But ultimately you don't want to wrap your entire application in a try-catch nor do you want to catch a general Exception. Try-catches generally slow down your application and a lot of times if you catch every general exception than it could be possible that you wouldn't know a bug exists until months or years later because the try-catch caused you to overlook it.

回答11:

I am all for catching specific known exceptions and handling state...but I use general catch exceptions to quickly localize problems and pass errors up to calling methods which handle state just fine. During development as those are caught, they have a place right next to the general exception and are handled once in release.

I believe one should attempt to remove these once the code goes into production, but to constantly be nagged during the initial code creation is a bit much.

Hence turn off (uncheck) the warning by the project settings as found in Microsoft.CodeQuality.Analyzers. That is found in the project settings under Code Analysis: