What's wrong with this code? (#3) - Answer

Re:

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MBA — Sat, 18 Dec 2004 18:54:00 GMT

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re: What's wrong with this code? (#3) - Answer

David Levine — Mon, 29 Nov 2004 12:46:00 GMT

Actually, looking at the InnerException is also error prone...there's no guaranteee that there is a single InnerException as there is no limit to the number of catch-wrap-throw's that occurred, so there is no way to determine in advance which InnerException you should examine. If you really want to look at the root cause you can use e.GetBaseException(), but again, there's no guarantee that this will be what you want - the ultimate root cause may be a null reference which gets translated into a DatabaseException by a technical layer.

This reinforces Eric's original point - if a method throws recoverable exceptions then it should be of a type other then System.Exception. It should be a specific type so that a caller is never forced to examine inner exceptions, only the outermost exception. The inner exceptions should provide context and clarify the situation.

As a side note, one could also argue that this ambiguity reflects a fundamental weakness in the current exception model and its implementation.

re: What's wrong with this code? (#3) - Answer

Eric Newton — Fri, 26 Nov 2004 00:13:00 GMT

Simplest is best...

if( e.InnerException is DatabaseException )
{ ... }

re: What's wrong with this code? (#3) - Answer

AT — Mon, 22 Nov 2004 06:22:00 GMT

Or the best one
if (e.InnerException is DatabaseException) ;-)

re: What's wrong with this code? (#3) - Answer

AT — Mon, 22 Nov 2004 06:20:00 GMT

Marcelo,
e.InnerException can be a null.

Much better is
(typeof(DatabaseException).IsInstanceOfType(e.InnerException))

re: What's wrong with this code? (#3) - Answer

Marcelo — Mon, 22 Nov 2004 01:34:00 GMT

You might want to change

if (e.InnerException.GetType() == typeof(DatabaseException))

to

if (typeof(DatabaseException).IsAssignableFrom(e.InnerException.GetType()))

This lets the called code throw a DatabaseException subclass in the future with more information, if they so wish, without breaking your code. Exact type comparison is rarely what people need in most situations...