What to return from a failed method and when to throw?

Question

I´ve lately been thinking about the things i´m returning from methods and I noticed that there are 4 different things i return when the method fails.

What bothers me about it, is that my code is not very consitent in this regard, so i wanted to ask about your "best practices".

So lets imagine a method that takes Foo and returns a list of Bar:

public IList<Bar> Method(Foo something);

Or to keep it more general:

public IBar Method(IFoo something);

The question is what do you return on what kind of failure. the options would be:

1. empty return type like: new List; or: new EmptyBar();
2. null
3. throw an exception
4. a special list value indicating failure like: new List{new FailureBar()}

I really hate option 4 so I´m mostly interessted to hear when you use the other 3 options and why

Answer 1

I'd choose between an empty list and an exception depending on the nature of the failure.

E.g.

If your database failed to connect - exception.

If your query didn't return results - empty list.