When to throw an exception?

[Guest]

Should I throw an exception in the following scenario?

I have a user class. The constructor takes one argument, the sessionid of
the user. This sessionid corresponds to an entry in a database. The
constructor calls the database with the passed in sessionid to retrieve the
user info. If the database call returns no rows, should I throw an exception
or return some kind of error code from the constructor?

 

[Jason Newell]

Larry,
I would recommend a Manager (or whatever name) class that you could call Manager.GetUser(userId). Using this method, you could simply return a User object or null. I would also mark the User constructor as internal based off your description.

Jason Newell, MCAD
Software Engineer

 

[Guest]

Still, should I throw an exception in the constructor of the user class which
can then be caught by the manager class?

 

[Bob Milton]

Larry,
Exceptions are rather expensive. It would be better to have the manager
class query the database. If there is no info, return null. Otherwise create
a class object and fill in the info that was returned.
Bob

 

[Jason Newell]

Echo what Bob said. Sorry for not being clear enough. The User class should not ever get created if the Manager does not find a matching record in the database.

Jason Newell, MCAD
Software Engineer

 

[Jon Skeet [C# MVP]]

Exceptions are rather expensive. It would be better to have the manager
class query the database. If there is no info, return null. Otherwise create
a class object and fill in the info that was returned.

I think it really depends on how exceptional it is to ask for something
which isn't there. If it indicates a significant programming error, and
the whole request should be aborted anyway, then there's nothing wrong
with using an exception IMO.

The "exceptions are expensive" idea is a bit of a myth in my view.
They're only going to end up being expensive in a significant way of
you throw many, many thousand exceptions. As a rough indication, my
laptop can throw of the order of 100,000 exceptions in a second. If
you're throwing anything *like* that number, you're using exceptions in
the wrong way in the first place, and have bigger worries than
performance, IMO.

 

[Patrice]

Is this something that could legitimely happens ? If no throw an exception.

In this partiuclar case I would rather expect to have a new line created for
me in the db assuming this is the first time I deal with this partiuclar
session but it all depends on what it's supposed to do...

Patrice