Catch most derived exceptions?

Question

In .NET, when catching exceptions, should I always catch derived exceptions (so not ArgumentException but the derived types)?

Also:

If I am asked to use error codes, would this be in the constructor like so?:

throw new Exception("4000", ex);

Or a custom exception type with an errorcode property? (This may get confusing with exception types like SqlException which have error codes mapping to SQL Server errors).

Thanks

Answer 1

1. Catch the broadest exception that you know how to handle.

   In general, this means you'll be catching a pretty specific exception. And some exceptions, like ArgumentExceptions, shouldn't be caught at all b/c they indicate a logic error as opposed to a runtime error. One place where I've found catching a broader exception useful is in the case of File I/O. An IOException can be a practical higher-level exception to catch.

2. If you are asked to use error codes, you could get away with using the message property of an exception to wrap it, but I would never use that as a reason to avoid throwing an appropriately typed exception. This is because there are two separate concerns here:

   a. The error code is there to provide a specific piece of information that can be looked up in the case of a failure in the field. It should never be used to discriminate between exception types programmatically b/c the language has a specific facility designed for that: exception types.

   b. The appropriately typed exception is there to provide a programmatic way of distinguishing between exceptions. The language is designed for it, use it. Don't ever throw a plain Exception.

   I would probably throw an error code into the Exception.Data collection. This avoids overwriting messages in Exception.Message that would otherwise be very helpful for diagnostic purposes.