C# - Rethrow an exception without setting it to an variable

Question

As all of you probably know, catching and re-throwing a exception in c# this way is evil, because it destroys the stack trace:

try
{
    if(dummy)
        throw new DummyException();
}
catch (DummyException ex)
{
    throw ex;
}

The right way to re-throw an exception without loosing the stack trace is this:

try
{
    if(dummy)
        throw new DummyException();
}
catch (DummyException ex)
{
    throw;
}

The only problem with this is that I get a lot of compilation warnings: "The variable 'ex' is declared but never used". If you have a lot of these, a useful warning may be hidden in the garbage. So, that's what I did:

try
{
    if(dummy)
        throw new DummyException();
}
catch (DummyException)
{
    throw;
}
catch(AnotherException ex)
{
    //handle it
}

This seems to work, but I'd like to know if there is any downside of re-throwing an exception that is not set to an variable. How does .net threats this?

Thanks in advance

Edit: I've changed my code a little bit to make clearer what I wanted to do, as some had misunderstood

Erm, why in the world do you throw an exception, catch it, and rethrow it all from the same method? — Cody Gray
– Cody Gray ♦, Commented May 2, 2011 at 15:13
@Cody Gray- I'm just guessing, here, but (giving the benefit of the doubt) I'm guessing he's just providing example code to show that he understands the difference between throw and throw ex. Not that he's actually generating an exception within a method- catching that exception- and then rethrowing it. I hope, anyway. — AllenG
– AllenG, Commented May 2, 2011 at 15:28
@Cody Gray, @AllenG is right, it's just an simple example to show what I'm trying to achieve, not a real code — Ortiga
– Ortiga, Commented May 2, 2011 at 15:50
Okay, good. That's what I was hoping, and why I posted that as a comment rather than an answer. You'd be surprised at the kind of craziness I see people do on here with throwing/catching exceptions. — Cody Gray
– Cody Gray ♦, Commented May 2, 2011 at 15:52

Cody Gray · Accepted Answer · 2019-07-04 09:13:32Z

9

I'd like to know if there is any downside of re-throwing an exception that is not set to an variable.

No, there's no downside at all. A variable is only needed if you want to reference the exception in your code, but since you don't need to do that with a throw statement, you don't need a variable at all.

And you have exactly the right idea in attempting to eliminate "noisy" compiler warnings. They have a tendency to bury important errors that you do want to fix, and getting a clean build is always important. The best solution is simply to rewrite the code to use a parameterless catch clause.

However, be aware that in 82% of cases that I see^*, it's a mistake to write code that uses throw at all. You generally shouldn't catch exceptions that you don't know how to handle and your only intended "handling" strategy is to rethrow them. There are cases where even using throw can reset the call stack, causing you to lose important debugging information. There are also better alternatives for logging exceptions to catching/rethrowing. You can find more information in the answers to these questions:

There's absolutely nothing wrong with letting exceptions bubble up and handling them all in a central place. The rule to keep in mind is that you shouldn't use exceptions for flow control. But there's nothing wrong with throwing an exception in low level code, and showing the user an error message higher up the stack in UI code. Read Microsoft's Best Practices for Handling Exceptions for some general tips.

_{^* Slightly more than the percent of statistics that are made up on the spot.}

edited Jul 4, 2019 at 9:13

answered May 2, 2011 at 15:55

Cody Gray♦

246k53 gold badges513 silver badges591 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Ortiga Over a year ago

thanks for your answer, that's exactly what I was looking for. I've already did a lot of mistakes in exception handling, but I think I'm getting a little better with it now. I usually handle them inside the method or one layer up, avoiding it going too many layers up. I usually throw an unchecked exception that bubble up and give a generic error message to the user. Is it a bad practice?

Cody Gray Over a year ago

@andre: There's absolutely nothing wrong with letting exceptions bubble up and handling them all in a central place. The rule to keep in mind is that you shouldn't use exceptions for flow control. But there's nothing wrong with throwing an exception in low level code, and showing the user an error message higher up the stack in UI code. You'd probably have to post real sample code for me to be any more specific. But you should read Microsoft's Best Practices for Handling Exceptions for some general tips.

Matt Greer · Accepted Answer · 2011-05-02 15:11:17Z

8

There is no downside to that. You are simply telling the compiler "I plan to catch this exception but I have no need for a reference to the actual exception", it doesn't affect how things are thrown or how exceptions work. Your latter example is the ideal way to do what you want, however if you are merely going to immediately throw; with nothing else whatsoever in the block, then why catch at all?

answered May 2, 2011 at 15:11

Matt Greer

62.1k18 gold badges127 silver badges124 bronze badges

Comments

Donut · Accepted Answer · 2011-05-02 15:10:17Z

5

If you're not doing anything with DummyException in the catch block (which you can't, since you haven't given it an identifier), why not get rid of the try/catch block entirely? E.g., just do this:

throw new DummyException();

Although at that point, I'd probably evaluate what you're trying to accomplish here and rethink your application architecture so as not to not rely on exception propagation in this manner.

answered May 2, 2011 at 15:10

Donut

113k20 gold badges136 silver badges147 bronze badges

6 Comments

AllenG Over a year ago

The reason you might catch DummyException specifically and just re-throw it is that you might be handling it at a higher (closer to the UI) level. And try/catch allows you to a) include a finally if you need one, and b) to catch multiple exceptions.

Matt Greer Over a year ago

@AllenG it's ok to have a try/finally which has no catch blocks. I don't know for sure but I wouldn't be surprised if the compiler optimized out his catch altogether.

AllenG Over a year ago

@MattGree: true: but you still need the try. I was referring to the suggestion to "get rid of the try/catch block entirely" which you wouldn't want to do- since you'd at least need the try.

Donut Over a year ago

@AllenG Good points worth considering; my answer was based off the OP's example (no other catch blocks and no finally).

Ortiga Over a year ago

@Donut, i gave this example because it shows my problem - not using the variable if I'm going to re-throw, what gives me a compiler warning. In a real code, I would have multiple catches and/or a finally block

|

Community · Accepted Answer · 2017-05-23 12:06:53Z

1

Why catch if your simply going to re-throw? Anyway, you may want to take a look at this previous discussion. Though not identical much of what is discussed is relevant:

Throw VS rethrow : same result?

edited May 23, 2017 at 12:06

CommunityBot

11 silver badge

answered May 2, 2011 at 15:11

Roja Buck

2,35416 silver badges26 bronze badges

1 Comment

Ortiga Over a year ago

thanks for the link. Answering your question, the code is just an example, I may re-throw one kind of exception but not others (if having multiple catch blocks)

Pop Catalin · Accepted Answer · 2011-05-02 15:12:38Z

1

Actually throwing an exception using "throw;" is a .Net best practice, because it preserves exception stack trace.

Throwing exceptions using "throw ex;" is considered a worst practice, because it looses the original stack trace and should be avoided.

answered May 2, 2011 at 15:12

Pop Catalin

63.1k24 gold badges90 silver badges116 bronze badges

1 Comment

Roja Buck Over a year ago

He does actually state that in the question... :)

AnotherOne · Accepted Answer · 2011-07-13 20:06:33Z

1

Why catch and rethrow like this? Why do people always have to assume they know every case? Answer: Database transactions.

If you have a better way to do this please speak up Dr. Proton, no offense taken. Keep in mind that there are a lot of different database systems in use but most of them support transaction control (begin/commit/rollback) and have C# interfaces.

pseudocode (a simplified case):

try
{
   db.beginTrans();

   db.doStuff();

   db.doOtherStuff();

   db.commitTrans();
}
catch
{
  db.rollbackTrans();
  throw;
}

Now, it is quite annoying to lose the detail of whether doStuff() or doOtherStuff() failed and I don't see any good reason why C# would toss the line number information in this case. But it seems to. Hence the googling and my subsequent arrival. If I am missing something please do tell.

answered Jul 13, 2011 at 20:06

AnotherOne

111 bronze badge

1 Comment

AnotherOne Over a year ago

Here is a post I found which offers some insight and a possible solution: weblogs.asp.net/fmarguerie/archive/2008/01/02/…

Collectives™ on Stack Overflow

C# - Rethrow an exception without setting it to an variable

6 Answers 6

2 Comments

Comments

6 Comments

1 Comment

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

2 Comments

Comments

6 Comments

1 Comment

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related