Should a business rule violation throw an exception?

Question

Should a business rule violation throw an exception?

Answer 1

No. It's part of normal conditional-handling logic in the program (and often just a disguised form of user error).

Answer 2

It depends on what the business rule is, IMO. I would venture to say "not usually" but I'd view it on a case-by-case basis. I don't think there is any one answer, as different business rules might warrant it while others might not.

Answer 3

Because of the way I do my validation and my use of LINQtoSQL for ORM, yes. If an entity fails validation on a business rule during the OnValidate method, the only way to notify the calling code is to throw an Exception. In this case, I throw a custom DataValidationException. Using the OnValidate method hook in a partial class implementation of the entity makes it possible for me to enforce validation on update/insert so only valid data gets saved to the database.

EDIT I should make it clear that I typically do validation of user input at the client so the persistence layer validation is typically more insurance and rarely, if ever, fails. I don't handle the client-side validation as exceptions, but rather with conditional logic.

Answer 4

Do you mean, for example, that a value is supposed to be in the range 0-99 but somehow ended up being 105?

If it's coming from the user it's a matter of validation. Whether it is handled using exceptions or not depends on the idioms of your language.

If it's coming from your data store then yes, it seems reasonable to throw an exception. It means you have bad data and you need to figure out how it got there and prevent it from happening again.

Answer 5

Throwing exceptions can be computationally intensive, they are outside of the norm. For example in .net you have performance counters that are incremented - that is a heavyweight acitivty and so not something you would want to do instead of a simple conditional.

Answer 6

It really depends on what it is and where it is.

If it's some data coming from the user then as levand said it's a matter of validation. Validation can turn up both successful and failed, both are expected options with clear further action scenarios.

If it's something like method execution errors it could be a better idea to throw an exception and stop right there before more harm is done (such as producing inconsistencies in the database).

It is often a matter of perspective and your application design.

Answer 7

Usualy I put the condition in a Specification object that implements

bool IsVerfiedBy(T entity);

So you can check the condition without exception.

If there is a place in your code where the specification should be verified beforehand, you can throw an exception because this is a prerequisit of you function.

For instance if your entity must be in a specific state before persistance, throw an exception when the specification is not verified, but use the specification before persisting so that the exception does not happen.

Answer 8

I would say not normally but I don't think you can say never.

For instance it depends on who/what is handling of the failed rule. If it is a user interface/user then I would use conditional logic to deal with the failure appropriatly. However if it is a business rule failure in, for instance, a faceless process that logs any errors to an event log which will be monitored by for a technical resource then an exception may be just as appropriate. In this later case an appropraitly named exception can be just as helpful as a nicely formatted message.

Answer 9

As an alternative view to most of the answers...

It can be useful to throw exceptions from the business logic, particularly if they are cuased by a failure in validation. If you are expecting an object and you get a null, it suggests that some problem has evaded detection in the user interface (or other interface). It may be completely valid to throw exceptions at this point. Indeed, you may decide to place this type of validation in the business logic when there are multiple interfaces.

Throwing exceptions in some languages / frameworks (I am thinking .NET) can be costly but this should not immediately worry you. It does mean that, at the name suggests, they are used for exceptional circumstances and not as part of the standard flow of a program. You certainly shouldn't throw an exception just to exit a method. You should also consider a graceful recovery where possible that may not include throwing an exception.

So, summing up... It depends...

Answer 10

First, a couple of quotes from chapter 18 of Applied Microsoft .NET Framework Programming (page 402) by Jeffrey Richter:

"Another common misconception is that an 'exception' identifies an 'error'."

"An exception is the violation of a programmatic interface's implicit assumptions."

If I'm inferring correctly from your question that a business rule violation would be data that falls outside a certain range (for example), this is an error that you could handle with a conditional as "ahockley" suggested. Based on the definition of an exception from Richter, the appropriate use of an exception would be if your code wasn't able to retrieve a business rule from whatever repository you're using. Being able to retrieve a business rule would be a reasonable implicit assumption for that interface to have, so an exception should be thrown if this assumption was violated.

One good example of Richter's first quote (exception != error) is the ThreadAbortException. If you call Response.Redirect(url) (in ASP.NET), a ThreadAbortException is thrown even though the redirect succeeds. Why? The implicit assumption of ASP.NET page execution is that a page will execute completely. Response.Redirect(url) violates this assumption, hence the exception.

Answer 11

Business rules should not throw an exception, unless they are used to validate parameters of some API (i.e.: checking requests validity) or in unit tests (i.e.: using business rules to simplify .NET unit tests).

Generally business rules output error and warning messages to the validation scope, though.

Answer 12

No Violating a business rule is a BUSINESS issue where an exception is a technical one. Violating a business rule is something that the system should regard as normal operation and for which it should have a programmed response, not an exception.