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

Here is an overview of the different methods:

• Find() - when you want to get an item by primary key. This will return null if it can't find an item. It will look in the context before going to the database (as pointed out by Yaron in the comments) which can be an important efficiency factor if you need to get the same entity multiple times while the same context is alive.

• Single() - when you expect exactly one item to be returned by a query. This will throw an exception if the query does not return exactly one item.

• SingleOrDefault() - when you expect zero or one items to be returned by a query (i.e. you are not sure if an item with a given key exists). This will throw an exception if the query does not return zero or one items.

• First() - when you expect one or more items to be returned by a query but you only want to access the first item in your code (ordering could be important in the query here). This will throw an exception if the query does not return at least one item.

• FirstOrDefault() - when you expect zero or more items to be returned by a query but you only want to access the first item in your code (i.e. you are not sure if an item with a given key exists)

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 the default value (which is null in case the given type is a reference type) 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.

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 you want to throw an exception if your query returns more then 1 record.