C# - Re-throwing Exceptions22 Sep 2013
In C# it is common that exceptions are re-thrown after some logging has taken place, or perhaps even to alter the exception information to be more user friendly. However there are two different ways of re-throwing exceptions in C# and care needs to be taken when doing so, as one method will loose the stack trace - making things a lot harder to debug.
Consider the following code:
This is pretty self-explanatory. We have a method that runs another method and catches any exceptions it may throw (in this case one will be thrown every time). In the catch block we examine the exception, perhaps do some logging and re-throw the exception for the caller to handle. Finally in Main the re-thrown exception is caught again and the stack trace is examined.
At first look there is nothing wrong with this code, it’s all pretty commonplace, nothing much to see here. However this is the output that we get:
You may or may not have noticed that this is not the full stack trace. We can see that the exception came from
Run(), however we can’t tell that in actual fact the exception originated from
DoSomething() at all. This may or may not cause problems when debugging as now instead of going straight to the route cause, you first have to go through
We lose the top of the stack trace because we used
Which essentially resets the stack trace to now start in that method. This makes sense as this is really the same as doing something like:
But what if we wanted to see the whole stack trace? Well instead of using
throw e; we just use:
With the updated catch block:
We get the output:
We now have the full story in the stack trace. We can see that the exception originated from the
DoSomething() method and passed through the
Run() method into
Main() - much more helpful when debugging.
I don’t see any situation when using
throw e; would be of any use at all. If you wanted to hide the stack trace then you would typically be throwing a completely new exception anyway - with a new message and perhaps other information to pass to the caller. If you didn’t want to hide the stack trace then throw; is the statement to use. Resharper even sees
throw e; as a problem and tries to replace it with the simple throw;.
Even so I bet this mistake has been made a lot of times by a lot of people. So remember if you are wanting to re-throw an exception, never use throw e; as it will loose your stack trace. Instead always use