Compare LINQ Queries for Single Items

Compare Linq Queries For Single Items

Four LINQ Queries for Returning Single Values

Using LINQ queries, there are several methods for querying a collection of records in a database to return a single result, these are:

• First()
• Single()
• FirstOrDefault()
• SingleOrDefault()

[edit: this article is about reference types, value types behave slightly different as is noted by Matt T Heffron in the comments.] 

They will each behave slightly different in cases where there are no results or more than 1 result to return.

So if there is a possibility that you could return zero results; you have to either check for an InvalidOperationException or null values. Personally I don’t like having to deal with exceptions for results; I would rather have to deal with null values. So using the chart I have a choice of FirstOrDefault or SingleOrDefault. Again I don’t want to have to deal with exceptions if the results are greater that 1; so that leaves me with FirstOrDefault.

What if your query will only return the 1 result, which do you choose?

Looking at the TSQL that is generated; there is only one difference.

First() and FirstOrDefault() will do:-

SELECT TOP(1)

Whereas Single() and SingleOrDefault() will do:-

SELECT TOP(2)

Why is this you ask?

It goes back to throwing exceptions. Doing a TOP(2) is the only way to determine whether there is more than 1 result; if there is it will throw the exception.

If you are returning zero results

As a side note it would be slightly more expensive to check for zero results by doing a count instead of handling a null value as the database will be doing a sub-query as the Count() operator generates the following TSQL:

SELECT[GroupBy1].[A1] AS [C1]
FROM ( SELECTCOUNT(1) AS [A1]
FROM [dbo].[MyTable] AS [Extent1]
WHERE 100 = [Extent1].[Id]) AS [GroupBy1]

returns 0

So it might be better to use FirstOrDefault() and check for nulls.

Happy coding!

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