How to avoid throwing vexing exceptions?
- by Mike
Reading Eric Lippert's article on exceptions was definitely an eye opener on how I should approach exceptions, both as the producer and as the consumer. However, I'm still struggling to define a guideline regarding how to avoid throwing vexing exceptions.
Specifically:
Suppose you have a Save method that can fail because a) Somebody else modified the record before you, or b) The value you're trying to create already exists. These conditions are to be expected and not exceptional, so instead of throwing an exception you decide to create a Try version of your method, TrySave, which returns a boolean indicating if the save succeeded. But if it fails, how will the consumer know what was the problem? Or would it be best to return an enum indicating the result, kind of Ok/RecordAlreadyModified/ValueAlreadyExists? With integer.TryParse this problem doesn't exist, since there's only one reason the method can fail.
Is the previous example really a vexing situation? Or would throwing an exception in this case be the preferred way? I know that's how it's done in most libraries and frameworks, including the Entity framework.
How do you decide when to create a Try version of your method vs. providing some way to test beforehand if the method will work or not? I'm currently following these guidelines:
If there is the chance of a race condition, then create a Try version. This prevents the need for the consumer to catch an exogenous exception. For example, in the Save method described before.
If the method to test the condition pretty much would do all that the original method does, then create a Try version. For example, integer.TryParse().
In any other case, create a method to test the condition.
