Entity Framework 4 Single() vs First() vs FirstOrDefault()

Question

I'm having a devil of a time finding a comparison of the different ways to query for a single item, and when to use each.

Does anyone have a link that compares all these, or a quick explanation as to why you would use one over the other? Are there still more operators that I am unaware of?

Thank you.

Answer 1

It's really very simple: Single returns a single item and throw an exception if there is either none or more than one item. First will return the first item or throw when there is no item. FirstOrDefault will return the first item or return null (in most cases) when there is no item.

This is the behavior the API is supposed to have. Note however that the underlying implementation could have a different behavior. While Entity Framework obeys this, a O/RM like LLBLGen can also return null when calling First which is a very strange thing. This was a very strange (and stubborn) decision by the designer IMO.

Answer 2

You missed SingleOrDefault() which will get a single value, throwing an exception if there is more than one item returned but returning a default value (normally a null) if there is no item returned by your query.

Answer 3

The four methods each have their place; Though you really only have two different operations.

• First - Expecting a result set that contains multiple items, give me the first item in that set.
• Single - Expecting a single result back, give me that item.

The xxxxOrDefault() version just adds on "I don't want to consider an empty result set to be an exceptional circumstance."

Answer 4

I always tend to use FirstOrDefault. If you really want to be picky with performance then you should use FirstOrDefault in EF. Under the covers SingleOrDefault uses top (2) in the query because, it needs to check if there is a second row that matches the criteria and if it does, it throws an exception. Basically in SingleOrDefault you are saying that u care to throw an exception if your query returns more then 1 record.