LINQ: When to use SingleOrDefault vs. FirstOrDefault() with filtering criteria

user_23199 0
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Consider the IEnumerable extension methods SingleOrDefault() and FirstOrDefault()

MSDN documents that SingleOrDefault:

Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.

whereas FirstOrDefault from MSDN (presumably when using an OrderBy() or OrderByDescending() or none at all),

Returns the first element of a sequence

Consider a handful of example queries, it's not always clear when to use these two methods:

var someCust = db.Customers
.SingleOrDefault(c=>c.ID == 5); //unlikely(?) to be more than one, but technically COULD BE

var bobbyCust = db.Customers
.FirstOrDefault(c=>c.FirstName == "Bobby"); //clearly could be one or many, so use First?

var latestCust = db.Customers
.OrderByDescending(x=> x.CreatedOn)
.FirstOrDefault();//Single or First, or does it matter?

Question

What conventions do you follow or suggest when deciding to use SingleOrDefault() and FirstOrDefault() in your LINQ queries?

linq-to-sql linq .net
16 Answers

If your result set returns 0 records:

  • SingleOrDefault returns the default value for the type (e.g. default for int is 0)
  • FirstOrDefault returns the default value for the type

If you result set returns 1 record:

  • SingleOrDefault returns that record
  • FirstOrDefault returns that record

If your result set returns many records:

  • SingleOrDefault throws an exception
  • FirstOrDefault returns the first record

Conclusion:

If you want an exception to be thrown if the result set contains many records, use SingleOrDefault.

If you always want 1 record no matter what the result set contains, use FirstOrDefault

user_5845930

For LINQ -> SQL:

SingleOrDefault

  • will generate query like "select * from users where userid = 1"
  • Select matching record, Throws exception if more than one records found
  • Use if you are fetching data based on primary/unique key column

FirstOrDefault

  • will generate query like "select top 1 * from users where userid = 1"
  • Select first matching rows
  • Use if you are fetching data based on non primary/unique key column
user_28533760

In my opinion FirstOrDefault is being overused a lot. In the majority of the cases when you’re filtering data you would either expect to get back a collection of elements matching the logical condition or a single unique element by its unique identifier – such as a user, book, post etc... That’s why we can even get as far as saying that FirstOrDefault() is a code smell not because there is something wrong with it but because it’s being used way too often. This blog post explores the topic in details. IMO most of the times SingleOrDefault() is a much better alternative so watch out for this mistake and make sure you use the most appropriate method that clearly represents your contract and expectations.

user_32701390

In essence this gives you some sort of validation to clean up your data, if you choose one over the other it will both give the data but SingleOrDefault will make you realize that when the data you're expecting should only have 1 result and spits out more 1 then you need to take a look at why your stored procedure or query resulted in such, duplicated items are never good in queries anyways.

user_4343470

I queried Google for the usage of the different methods on GitHub. This is done by running a Google search query for each method and limiting the query to the github.com domain and the .cs file extension by using the query "site:github.com file:cs ..."

It seems that the First* methods are more commonly used than the Single* methods.

| Method               | Results |
|----------------------|---------|
| FirstAsync           |     315 |
| SingleAsync          |     166 |
| FirstOrDefaultAsync  |     357 |
| SingleOrDefaultAsync |     237 |
| FirstOrDefault       |   17400 |
| SingleOrDefault      |    2950 |
user_24705240
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