LINQ: Understand the difference between Single vs SingleOrDefault vs First vs FirstOrDefault extension methods

Single(), SingleOrDefault(), First() and FirstOrDefault() are extension methods of the Enumerable class.

“These Extension methods are static method that we can call from an instance object which implement IEnumerable interface.”

Many people get confused about the difference between Single, SingleOrDefault, First, and FirstOrDefault extension methods in LINQ.

Let’s understand the difference between all these methods:

Single(): This method searches for single instances in a collection which matching a condition. If the collection has 0 or more than one matching element, we get an exception.

This method should be used if you are sure that there will exactly 1 element, for example you can use Single with primary column of an object.

SingleOrDefault(): This method returns a single, specific element in a collection, or a default value if that element is not found. If there is no matching element found then by default it’ll return null value.

However this method will throw exception if there is more than one matching element in the collection.

This method should be used if 0 or 1 matching elements are expected.

First(): This method locates the first matching element in a collection. If the collection has no matching element, we get an exception.

This method should be used when more than one element is expected and you want only the first.

FirstOrDefault(): This method is almost the same as First. The difference is how it handles empty collections. If a collection is empty, it returns the default value for the type. This method eliminates the exceptions.

This method should be used when more than one element is expected and you want only the first element and with that this method provides you assurance that there will be no exception if there is no matching element found.

So I hope you got the fair idea about these methods, still need more clarification? Don’t worry; See below example for more understanding:

Example:

First we’ll create an object of employee.

public class Employee {     public int Id { get; set; }     public string Name { get; set; }     public string City { get; set; } }

Then we’ll write a method to return list of employees (for example we are using hard-coded data, in real scenario data will be populated from DB).

/// <summary> /// Method return list of employees /// </summary> /// <returns></returns> private List<Employee> GetEmployees() {     var employeeList = new List<Employee>();     for (var i = 1; i <= 5; i++)     {          employeeList.Add(new Employee                              {                                  Id = i,                                  Name = "Name" + i,                                  City = "Gurgaon"                              });     }     return employeeList; }

Now we’ll test these methods with following different scenarios:

Id = 1, i.e. only one employee exist against this Id

City = Gurgaon, i.e. more than one employee for this city

Id = 100: No employee exists against this Id

Populate all employees in a list as: var employeeList = GetEmployees();

Method: Single()

Statement 1: employeeList.Single(e => e.Id == 1); Result: Employee object returned having Id as 1. This method is best is this scenario. Statement2: employeeList.Single(e => e.City == "Gurgaon"); Result: There are more than one matching element, it’ll throw exception: InvalidOperationException: Sequence contains more than one element Statement3: employeeList.Single(e => e.Id == 100); Result: There is no matching employee with Id 100, it’ll throw exception: InvalidOperationException: Sequence contains no elements

Method: SingleOrDefault()

Statement 1: employeeList.SingleOrDefault(e => e.Id == 1); Result: Employee object returned having Id as 1. Statement2: employeeList.SingleOrDefault(e => e.City == "Gurgaon"); Result: There are more than one matching element, it’ll throw exception: InvalidOperationException: Sequence contains more than one element Statement3: employeeList.SingleOrDefault(e => e.Id == 100); Result: There is no matching employee with Id 100; it’ll return default null value.

Method: First()

Statement 1: employeeList.First(e => e.Id == 1); Result: Employee object returned having Id as 1. Statement2: employeeList.First(e => e.City == "Gurgaon"); Result: There are more than one matching element, it’ll return first employee object from list. Statement3: employeeList.First(e => e.Id == 100); Result: There is no matching employee with Id 100; it’ll throw exception InvalidOperationException: Sequence contains no elements.

Method: FirstOrDefault()

Statement 1: employeeList.FirstOrDefault(e => e.Id == 1); Result: Employee object returned having Id as 1. Statement2: employeeList.FirstOrDefault(e => e.City == "Gurgaon"); Result: There are more than one matching element, it’ll return first employee object from list. Statement3: employeeList.FirstOrDefault(e => e.Id == 100); Result: There is no matching employee with Id 100; it’ll return default null value. This method is good in the case you are not sure whether matching element exist or not.

Hope that clears all the confusions, your comment or suggestions will be highly appreciated.