Is there a benefit to JUST a “throw” in a catch?

问题:

Been having a "heated debate" with a colleague about his practice of wrapping most of his functions in a try/catch but the catch has JUST a "throw" in it e.g.

Private sub foo()
    try
        'Do something'
    catch
        throw 'And nothing else!'
    End Try
End Sub

My thought was to not even bother (assuming you don't need to do anything at this point) - the exception would bubble to the next exception handler in a parent member.

The only argument that sounded plausible was that sometimes exceptions weren't caught and your code stopped (in debug mode) with the current line highlighted in green...and that this may be something to do with multiple threads? Best practice does state "an exception handler for each thread" but mostly we work single-threaded.

The good thing may be it could be useful in debug mode to not suddenly pop out to a parent member (yes, Joel!) - you'd move to the "throw" statement and be able to examine your locals. But then your code would be "littered with try/catch/throws" (to quote another thread here)?

And what sort of overhead would be involved in adding try/catch/throws everywhere if no exception occurs (i.e. should you avoid try/catches in tight loops)?

回答1:

The reason you have a lone throw inside a catch rather than throwing a new exception is because this causes the original stack trace/exception data to be preserved. And one reason you might do this is because you can now set a break-point there for debugging.

回答2:

Microsoft recommends not to catch an exception when the only thing you do is to rethrow it immediately (i dont remember the source for now). Your code should only catch exceptions that you want to handle for clean up things or similar actions.

So generally its not a good practice to catch and rethrow an exception.

Reasons for catching and replacing it with another exception might be

• Logging
• Hiding sensitive information from the caller (Stacktrace, exception details)

And for debugging you might want to change your "Break when an exception is:"-Handler (Press Ctrl+Alt+e) the value "thrown" on selected CLR Exceptions.

You might want to take a look at the entlib exception handler block (EHB), with which you can establish a pattern on how to deal with exceptions in your code.

Regarding your question on performance i think its not a propblem to have many try/catch blocks in your code but you will get performance hits when your code raises and catches many exceptions.

回答3:

I would only ever do this while debugging an issue - and I'd remove the code again before checking in. It can occasionally be handy to put a breakpoint in to stop at a particular stack level if an exception is thrown. Beyond that though - no.

回答4:

In practice, my thought is, if you don't intend to handle the error, don't catch it.

回答5:

One effect of using a catch and immediate rethrow is that any inner "Finally" blocks will run before the "Catch" occurs (which will in turn be before the exception propagates). This is relevant in two scenarios:

1. If an exception is ultimately unhandled, it's possible that the unhandled-exception trap may quit the application without running any "finally" blocks. Doing a catch and immediate rethrow will ensure that all "finally" blocks within the catch will execute, even if the exception ends up ultimately being unhandled.
2. It is possible for code in vb.net, and possibly other languages, to act upon an exception before any finally blocks are run. Using a "try" block with a catch-and-immediate-rethrow will cause the "finally" blocks within that catch block to run before any outer "try" blocks get their first look at the exception.

An additional caveat with catch-and-immediate-rethrow: for some reason, a catch and immediate rethrow will trash the stack trace's line number for the function call that caused the exception. I don't know why the current function's entry in the stack trace can't be left alone in that case, but it isn't. If one isn't using a .pdb file to get line-number information, this isn't an issue, but if one wants to use such information, it can be annoying.

Generally, the effects mentioned above aren't desirable, but there are occasions when one or both of the first two effects they may be useful, and the third effect tolerable. In those cases, a catch with immediate rethrow may be appropriate, though the reason for it should be documented.

回答6:

Doing it always by default looks like bad design. But there might be reasons for catching and throwing, for example it you want to throw a different exception.

回答7:

You do not need a catch clause to catch exceptions in the Visual Studio debugger. Choose Debug > Exceptions, and select which exceptions you want to catch, all of them if necessary.

回答8:

If you catch an exception and replace it with another exception, you should typically wrap the original exception in the new one. This is usually done by passing the old exception into the new one's constructor. That way you can dig in as much as necessary to figure out what happened. The main case when you wouldn't is when you need to hide data for security reasons. In these cases, you should try to log the exception data before you clear it out.

The rationale I have seen for wrapping exceptions with new ones, rather than just letting them bubble up the stack, is that exceptions should be at the same symantic level as the methods they are coming from. If I call AuthenticateUser, I don't want to see an SQL exception. Instead, I should see some exception whose name tells me the authentication task could not be completed. If I dig into this exception's inner exceptions, I could then find the SQL exception. Personally, I am still weighing the pros and cons of doing this.

回答9:

Yes, it's handy for putting a breakpoint in the catch.

An alternate and cleaner way is to breakpoint in the constructor of the object you're throwing. You're seeing the program state at a point closer to the source of the error.

回答10:

Since there is zero error handling, this catch is useless. If there was logging or some cleanup done sure, but in this situation I'd get rid of the try/catch.

回答11:

This can also be useful if you need to inspect something about the exception, and do something for one circumstance or throw it for other circumstances. For instance, if you need to inspect the error number in a SQLException. You can perform a certain action if the error number is one you're prepared to handle. For others you can simply "throw" it so the stack trace is preserved, as mentioned above.