Rethrowing Exceptions Is a Dangerous Business
July 9, 2007
Problem: in Visual Studio's debugger, you've landed on an Exception statement. You look at the stack trace but it just points back to a custom exception class you've created. But you know that code is good, it can't be throwing the error. What's going on here?
Solution: this is an easy mistake to make. I've run it across it in code from a number of people, and recall making the mistake myself at one point. The problem is most likely due to nested throws to a new custom exception.
Let's say you have a custom exception class, CustomError, inheriting from System.Exception. Further, let's say that you have an application that calls a function libraryFunction() which in turn calls another function innerFunction(). Both of them take all exceptions and wrap them in CustomError (which perhaps does some logging that you don't get with System.Exception). In psuedocode, you have something like:
private class CustomError : Exception
{
public CustomError() : base() { }
public CustomError(string message) { this.Message = message; }
}
private void libraryFunction()
{
try
{
...
innerFunction();
...
}
catch (Exception ex)
throw new CustomError(ex.Message);
}
private void innerFunction()
{
try
{
...
}
catch (Exception ex)
throw new CustomError("Something awful happened");
}
Now, if you're debugging your application and you run across an exception coming from libraryFunction(), the stack trace will report that the error came from CustomError instead of innerFunction(). You won't be able to see that it came from line x in innerFunction(). Thankfully there are several ways that we can and should clean up this code.
1. Do we really need try in innerFunction()?
That is, is there really any point to catching the exception within innerFunction()? There very well could be some good point, but often times it is just out of habit. Maybe its best to just let the exception propagate directly through to libraryFunction(). Throwing exceptions is expensive, in terms of performance. You should do so only when circumstances clearly justify it.
2. Use inner exception
One way to avoid losing the stack trace is to use the InnerException property. You might want to add a constructor in CustomError that takes the exception itself as an argument, along with a string for a more customized message:
public CustomError(Exception inner, string message)
{
this.InnerException = inner;
this.Message = message;
}
Now you have both the original stack trace and the possibility to customize your error message.
3. Rethrow without new
If for some reason you really need to throw CustomError in innerFunction(), then you'll have better performance — and no loss of information — if you specifically catch CustomError and rethrow it as is, without using new:
private void libraryFunction()
{
try
{
...
innerFunction();
...
}
catch (CustomError)
throw;
catch (Exception ex)
throw new CustomError(ex, "Another awful thing happened");
}
Now, if I had fully read the Exception Management Architecture Guide, I'm sure I could have explained this all in more technically precise language. Then again, maybe (hopefully) its more intelligible in this form =).