Difference between First & FirstOrDefault and Single & SignleOrDefault

	Single()	SingleOrDefault()	First()	FirstOrDefault()
Description	Returns a single, specific element of a sequence	Returns a single, specific element of a sequence, or a default value if that element is not found	Returns the first element of a sequence	Returns the first element of a sequence, or a default value if no element is found
Exception thrown when	There are 0 or more than 1 elements in the result	There is more than one element in the result	There are no elements in the result	Only if the source is null (they all do this)
When to use	If exactly 1 element is expected; not 0 or more than 1	When 0 or 1 elements are expected	When more than 1 element is expected and you want only the first	When more than 1 element is expected and you want only the first. Also it is ok for the result to be empty

Examples

First we create an Employee table for querying. We will test with three difference scenarios:

• Employeeid = 1: Only one employee with this ID
• Firstname = Robert: More than one employee with this name
• Employeeid = 10: No employee with this ID

Employeeid	Lastname	Firstname	Birthdate
1	Davolio	Nancy	12/8/1948 12:00:00 AM
2	Fuller	Andrew	2/19/1952 12:00:00 AM
3	Leverling	Janet	8/30/1963 12:00:00 AM
4	Peacock	Margaret	9/19/1937 12:00:00 AM
5	Buchanan	Robert	3/4/1955 12:00:00 AM
6	Suyama	Michael	7/2/1963 12:00:00 AM
7	King	Robert	5/29/1960 12:00:00 AM
8	Callahan	Laura	1/9/1958 12:00:00 AM
9	Dodsworth	Anne	1/27/1966 12:00:00 AM

Single()

Statement	Employee.Single(e => e.Employeeid == 1)
Expected Result	There is only one record where Employeeid == 1. Should return this record.
Actual Result	Employeeid 1 Lastname Davolio Firstname Nancy Birthdate 12/8/1948 12:00:00 AM

Statement	Employee.Single(e => e.Firstname == "Robert")
Expected Result	There are multiple records where Firstname == Robert. Should fail.
Actual Result	InvalidOperationException: Sequence contains more than one element

Statement	Employee.Single(e => e.Employeeid == 10)
Expected Result	There is no record with Employeeid == 10. Should fail.
Actual Result	InvalidOperationException: Sequence contains no elements

SingleOrDefault()

Statement	Employee.SingleOrDefault(e => e.Employeeid == 1)
Expected Result	There is only one record where Employeeid == 1. Should return this record.
Actual Result	Employeeid 1 Lastname Davolio Firstname Nancy Birthdate 12/8/1948 12:00:00 AM

Statement	Employee.SingleOrDefault(e => e.Firstname == "Robert")
Expected Result	There are multiple records where Firstname == Robert. Should fail.
Actual Result	InvalidOperationException: Sequence contains more than one element

Statement	Employee.SingleOrDefault(e => e.Employeeid == 10)
Expected Result	There is no record with Employeeid = 10. Should return default value.
Actual Result	null

First()

Statement	Employee.OrderBy(e => e. Birthdate) .First(e => e.Firstname == "Robert")
Expected Result	There are multiple records where Firstname == Robert. Should return the oldest one.
Actual Result	Employeeid 5 Lastname Buchanan Firstname Robert Birthdate 3/4/1955 12:00:00 AM

Statement	Employee.OrderBy(e => e. Birthdate) .First(e => e.Employeeid == 10)
Expected Result	There is no record with Employeeid = 10. Should fail.
Actual Result	InvalidOperationException: Sequence contains no elements

FirstOrDefault()

Statement	Employee.OrderBy(e => e. Birthdate) .FirstOrDefault(e => e.Firstname == "Robert")
Expected Result	There are multiple records where Firstname == Robert. Should return the oldest one.
Actual Result	Employeeid 5 Lastname Buchanan Firstname Robert Birthdate 3/4/1955 12:00:00 AM

Statement	Employee.OrderBy(e => e. Birthdate) .First(e => e.Employeeid == 10)
Expected Result	There is no record with Employeeid = 10. Should return default value.
Actual Result	null