LINQ First vs FirstOrDefault vs Single vs SingleOrDefault

Below is a chart explaining the difference between them and examples of each scenario.

	First()	FirstOrDefault()	Single()	SingleOrDefault()
Description	Return first element of the sequence elements	Return first element of the sequence elements or default value if no element is found	Return a single element of a sequence	Return a single element of a sequence, or a default value if that element is not found\
Exception Thrown when	There are no elements in the result	Only if the source is null	There are 0 or more than 1 elements in the result	There is more than one element in the result
When to use	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	If exactly 1 element is expected not 0 or more than 1	When 0 or 1 element is expected

Example

First, we create an Employee table for querying. We will test with different scenarios

Emp_Id	FirstName	LastName	BirthDay
1	Rahul	Sharma	12/8/1948 12:00:00 AM
2	Virat	Kohli	2/19/1952 12:00:00 AM
3	Yusuf	Pathan	8/30/1963 12:00:00 AM
4	Sudeep	Tyagi	9/19/1937 12:00:00 AM
5	Rahul	Kumar	3/4/1955 12:00:00 AM
6	Umesh	Yadav	7/2/1963 12:00:00 AM
7	Vinay	Ojha	5/29/1960 12:00:00 AM
8	Pankaj	Singh	1/9/1958 12:00:00 AM
9	Varun	Aaron	1/27/1966 12:00:00 AM

1) First()

Statement: Employee.OrderBy(e => e. BirthDay).First(e => e.FirstName == "Rahul")
Expected Result:  There are multiple records where FirstName == Rahul. It will return the old one.
Actual Result

Emp_Id	5
FirstName	Rahul
LastName	Kumar
Birthdate	3/4/1955 12:00:00 AM

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

2) FirstOrDefault()

Statement: Employee.OrderBy(e => e. BirthDay) .FirstOrDefault(e => e.FirstName == "Rahul")
Expected Result:There are multiple records where FirstName == Rahul. It will return the old one. Actual Result:

Emp_Id	5
FirstName	Rahul
LastName	Kumar
Birthdate	3/4/1955 12:00:00 AM

Statement : Employee.OrderBy(e => e. BirthDay).FirstOrDefault(e => e.Emp_Id == 10)

Expected Result: There is no record with Empl_Id = 10. Should return the default value.

Actual Result: null

3) Single()

Statement: Employee: Single (e => e. Emp_Id == 1)

Expected Result: There is only one record where Emp_Id equal to 1
Actual Result:

Emp_Id	1
FirstName	Rahul
LastName	Sharma
Birthdate	12/8/1948 12:00:00

Statement: Employee.Single(e => e.FirstName == "Rahul")
Expected Result: There are multiple records where FirstName == Rahul it should fail.
Actual Result: InvalidOperationException: Sequence contains more than one element

Statement: Employee.Single(e => e.Emp_Id==10)

Expected Result: There is no record with Emp_Id equal to 10 it will fail

Actual Result: InvalidOperationException: Sequence contains no elements

4) SingleOrDefault()

Statement: Employee.SingleOrDefault(e => e.Emp_Id == 1) 
Expected Result: There is only one record where Emp_Id == 1.
Actual Result

Emp_Id	1
FirstName	Rahul
LastName	Sharma
Birthdate	12/8/1948 12:00:00 AM

Statement: Employee.SingleOrDefault(e => e.FirstName == "Rahul")
Expected Result: There are multiple records where FirstName == Rahul, it will fail.
Actual Result: InvalidOperationException: Sequence contains more than one element 

Statement: Employee.SingleOrDefault(e => e.Emp_Id == 10) 
Expected Result: There is no record with Emp_Id = 10 it will return the default value.
Actual Result: null

I hope this article is useful for you. Look forward for your comments and feedback.