Creating property which may throw IndexOutOfRangeException

Question

I have a class containing a collection of items. For convenience I've provided GetCurrentItem which is implemented by

public Type GetCurrentItem
{
     get { return this.items[this.items.Count - 1]; }  
}

which will throw an exception if there are no items in the list.

Should I let the exception be thrown or should I return null? If this was an API I handed to you, what would you expect? The exception or null? Is there a better way to handle this?

Answer 1

As to which is more correct? As Kirk's comment suggests: it depends. Sometimes a null makes logical sense and sometimes an exception is better suited if no default is reasonable. One thing I try to do is think of "is calling GetCurrentItem a logical failure or a safe thing?"

If it is a failure to call GetCurrentItem when there are none, then throwing an exception is the correct course. For example, if your collection has a HasCurrent or IsEmpty property where someone could examine the result before calling GetCurrentItem, then they should "know better". But if the current item is null is a correct logical way of using your class, then by all means design it that way. Either way, I'd document the behavior in the code comments to let users know of expected behavior.

I will say this though, exposing the ArgumentOutOfRange exception may be bleeding implementation details. That is, if the user of this class has no idea that the inner structure is an array or List<T>, then don't bleed out that exception, but catch it, wrap it, and throw a more meaningful one (custom, or something like InvalidOperationException).

Since they're not really directly passing in an argument, them getting an ArgumentOutOfRange exception could be confusing :-)