IndexNotFoundException versus NullReferenceException

Question

I have the following code that attempts to catch a null reference. It then throws an exception with a clearer reason for the error specified in the message property.

What type of exception should it throw? An IndexOutOfRangeException?

var existing = this.GetByItemId(entity.ItemId); // int or long
if (existing == null)
{
    throw new IndexOutOfRangeException("The specified item does not exist.");
}

var price = existing.Price;

or a NullReferenceException?

var existing = this.GetByItemId(entity.ItemId);
if (existing == null)
{
    throw new NullReferenceException("The specified item does not exist.");
}

var price = existing.Price;

or, should we have just let the exception run its course?

var existing = this.GetByItemId(entity.ItemId);
var price = existing.Price; // NullReferenceException coming your way

The reason we tend not to do this last option, is that the default NullReferenceException is light on detail and just states

Object reference not set to an instance of an object.

Which, to be honest, could quite well be the most unhelpful error message in C#.

Answer 1

I would use a custom exception for this (some thing like ItemNotFoundException).

A NullReferenceExceptionor IndexOutOfRangeException might be thrown by something else inside of this.GetByItemId() or in the Framework somewhere.

The caller might wish to perform a follow up action if the item does not appear in the collection (e.g. adding it). Using your own exception allows the caller to catch that exception specifically and react accordingly.