Quick learning about Single, SingleOrDefault, First and FirstOrDefault

Hello readers, I am sharing post about Single() vs SingleOrDefault() vs First() vs FirstOrDefault(). Here, you can find  difference between Single() vs SingleOrDefault() vs First() vs FirstOrDefault() , When to use Single(), SingleOrDefault(), First(), FirstOrDefault() and different type of scenario with example.

Single()

It returns a single, specific element of a sequence if element match found. If none or more than one match found for that element in the collection then an exception is thrown.

When to use: If exactly 1 element is expected; not 0 or more than 1.

Examples

Following the Student Entity where we will perform query on that.

StudentId	Firstname	Lastname	DOB
1	Alok	Singh	12/8/1985
2	Anil	Singh	2/19/1984
3	Sachin	Kinra	8/30/1996
4	Cris	Moris	9/19/1981
5	Sahil	Joshi	3/4/1995
6	Rahul	Maurya	7/2/1969
7	Raj	Joshi	5/29/1984
8	King	George	1/9/1980
9	Michel	Jonhsan	1/27/1965

Query 1	Student.Single(e => e.StudentId == 1)
Expected Result	There is one record where StudentId == 1. It will return this record.
Result	StudentId Firstname Lastname DOB 1 Alok Singh 12/8/1985

Query 2	Student.Single(e => e.Lastname == "Joshi")
Expected Result	There are multiple records where Lastname == Joshi. Should fail.
Actual Result	InvalidOperationException: Sequence contains more than one element

Query 3	Student.Single(e => e.StudentId == 15)
Expected Result	There is no record with StudentId == 15. Should fail.
Actual Result	InvalidOperationException: Sequence contains no elements

SingleOrDefault()

It returns a single specific element from a collection of elements if there is any match found. If no match is found for that element in the collection, A default value is returned. If there is more than one match found for that element in the collection then an exception is thrown.

When to use: When 0 or 1 elements are expected.

Examples

Following the Student Enityt where i will to perform query on that.

StudentId	Firstname	Lastname	DOB
1	Alok	Singh	12/8/1985
2	Anil	Singh	2/19/1984
3	Sachin	Kinra	8/30/1996
4	Cris	Moris	9/19/1981
5	Sahil	Joshi	3/4/1995
6	Rahul	Maurya	7/2/1969
7	Raj	Joshi	5/29/1984
8	King	George	1/9/1980
9	Michel	Jonhsan	1/27/1965

Query 1	Student.SingleOrDefault(e => e.StudentId == 1)
Expected Result	There is only one record where StudentId == 1. Should return this record.
Actual Result	StudentId Firstname Lastname DOB 1 Alok Singh 12/8/1985

Query 2	Student.SingleOrDefault(e => e.Lastname == "Joshi")
Expected Result	There are multiple records where Lastname == Joshi. Should fail.
Actual Result	InvalidOperationException: Sequence contains more than one element

Query 3	Student.SingleOrDefault(e => e.StudentId == 15)
Expected Result	There is no record with StudentId = 15. Should return default value.
Actual Result	null

First()

It returns First element of a sequence or a value from collection of elements. If there are no elements in the result than it will throws InvalidOperationException.

When to use: When more than 1 element is expected and you want only the first .

Examples

Following the Student Enityt where i will to perform query on that.

StudentId	Firstname	Lastname	DOB
1	Alok	Singh	12/8/1985
2	Anil	Singh	2/19/1984
3	Sachin	Kinra	8/30/1996
4	Cris	Moris	9/19/1981
5	Sahil	Joshi	3/4/1995
6	Rahul	Maurya	7/2/1969
7	Raj	Joshi	5/29/1984
8	King	George	1/9/1980
9	Michel	Jonhsan	1/27/1965

Query 1	Student.OrderBy(e => e. DOB) .First(e => e.Lastname == "Joshi")
Expected Result	There are multiple records where Lastname == Joshi. Should return the oldest one.
Actual Result	StudentId Firstname Lastname DOB 7 Raj Joshi 12/8/1985

Query 2	Student.OrderBy(e => e. DOB) .First(e => e.StudentId == 15)
Expected Result	There is no record with StudentId = 15. Should fail.
Actual Result	InvalidOperationException: Sequence contains no elements

FirstOrDefault()

It returns the first element of a sequence or default value of underlying type of generic collection. It does not throw InvalidOperationException if no element found.

When to use: When more than 1 element is expected and you want only the first. Also it is fine for the result to be empty.

Examples

Following the Student Enityt where i will to perform query on that.

StudentId	Firstname	Lastname	DOB
1	Alok	Singh	12/8/1985
2	Anil	Singh	2/19/1984
3	Sachin	Kinra	8/30/1996
4	Cris	Moris	9/19/1981
5	Sahil	Joshi	3/4/1995
6	Rahul	Maurya	7/2/1969
7	Raj	Joshi	5/29/1984
8	King	George	1/9/1980
9	Michel	Jonhsan	1/27/1965

Query 1	Student.OrderBy(e => e. DOB) .FirstOrDefault(e => e.Lastname == "Joshi")
Expected Result	There are multiple records where Lastname == Joshi. Should return the oldest one.
Actual Result	StudentId Firstname Lastname DOB 7 Raj Joshi 12/8/1985

Query 2	Student.OrderBy(e => e. DOB) .FirstOrDefault(e => e.StudentId == 15)
Expected Result	There is no record with StudentId = 15. Should return default value.
Actual Result	null