LINQ Single vs First

Question

LINQ:

Is it more efficient to use the Single() operator over First() when ever I know for certain that the query will return a single record?

Is there a difference?

Answer 1

If I recall, Single() checks if there is another element after the first one (and throws an exception if it's the case), while First() stops after getting it. Both throw an exception if the sequence is empty.

Personnally, I always use First().

Answer 2

Single will throw an exception if it finds more than one record matching the criteria. First will always select the first record from the list. If the query returns just 1 record, you can go with First().

Both will throw an 'InvalidOperationException' exception if the collection is empty. Alternatively you can use SingleOrDefault(). This won't throw an exception if the list is empty

Answer 3

If you don't specifically want an exception thrown in the event that there is more than one item, use First.

Both are efficient, take the first item. First is slightly more efficient because it doesn't bother checking to see if there is a second item.

The only difference is that single expects there to be only one item in the enumeration to begin with, and will throw an exception if there are more than one. You use .single if you specifically want an exception thrown in this case.

Answer 4

They are different. Both of them assert that the result set is not empty, but single also asserts that there is not more than 1 result. I personally use Single in cases where I only expect there to be 1 result just because getting more than 1 result back is an error and probably should be treated as such.

Answer 5

There is a subtle, semantic difference between these two methods.

Use Single to retrieve the first (and only) element from a sequence that should contain one element and no more. If the sequence has more than on element your invocation of Single will cause an exception to be thrown since you indicated that there should only be one element.

Use First to retrieve the first element from a sequence that can contain any number of elements. If the sequence has more than on element your invocation of First will not cause an exception to be thrown since you indicated that you only need the first element in the sequence and do not care if more exist.

If the sequence contains no elements both method calls will cause exceptions to be thrown since both methods expect at least one element to be present.

Answer 6

If you're expecting a Single record, it's always good to be explicit in your code.

I know others have written why you use one or the other, but I thought I'd illustrate why you shouldn't use one, when you mean the other.

In my code, I will typically use FirstOrDefault() and SingleOrDefault() but that's a different question.

An example, a table that stores Customers in different languages with a Composite Key ( ID, Lang ):

DBContext db = new DBContext();
Customer customer = db.Customers.Where( c=> c.ID = 5 ).First();

This code above is an easy to make logic error ( can be difficult to catch ). It will return more than one record ( assuming you have the customer record in multiple languages ) but you will always just return the first one... which may work sometimes... but not others.

Since you always want to return a Single Customer use Single();

The following would throw an exception ( which is what you want in this case ):

DBContext db = new DBContext();
Customer customer = db.Customers.Where( c=> c.ID = 5 ).Single();

Then, you simply hit yourself on the forehead and say to yourself... OOPS! I forgot the language field! Following is the correct version:

DBContext db = new DBContext();
Customer customer = db.Customers.Where( c=> c.ID = 5 && c.Lang ="en" ).Single();

First is useful in the following scenario:

DBContext db = new DBContext();
NewsItem newsitem = db.NewsItems.OrderByDescending( n => n.AddedDate ).First();

It will get ONE object, and you're using sorting to get the more recent one.

Answer 7

A lot of people I know use FirstOrDefault(), but I tend to use SingleOrDefault() more because often it would be some sort of data inconsistency if there were more than one. This is dealing with LINQ-to-Objects, though.