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 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 should NOT use one, when you mean the other.

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

Take, for example, a table that stores Customers in different languages using a Composite Key ( ID, Lang ):

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

This code above introduces a possible logic error ( difficult to trace ). It will return more than one record ( assuming you have the customer record in multiple languages ) but it will always return only the first one… which may work sometimes… but not others. It’s unpredictable.

Since your intent is 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 return ONE object, and since you’re using sorting, it will be the most recent record that is returned.

Using Single() when you feel it should explicitly always return 1 record will help you avoid logic errors.