C#

First, FirstOrDefault, Single, SingleOrDefault In C#

For people who are new to LINQ, it is difficult to understand the difference between First, FirstOrDefault, Single, and SingleOrDefault. In this blog, I will explain what to use and when.

I will take a simple example to make you understand practically how these methods work.

Consider a class Employee with properties as Id, Name, and Department.

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

I have a list of Employees.

List<Employee> employeeList = new List<Employee>()
{
    new Employee() { Id = 1, Name = "Sunny", Department = "Technical" },
    new Employee() { Id = 2, Name = "Pinki", Department = "HR" },
    new Employee() { Id = 3, Name = "Tensy", Department = "Finance" },
    new Employee() { Id = 4, Name = "Bobby", Department = "Technical" },
    new Employee() { Id = 5, Name = "Sweety", Department = "HR" }
};

First()

  1. It returns the first element of a sequence.
  2. It throws an error when there is no element in the result, or the source is null.
  3. We should use it if more than one element is expected and you want only the first element.

Example 1

var result = employeeList.First();

Will return

new Employee() { Id = 1, Name = "Sunny", Department = "Technical" }

Example 2

var result = employeeList.First(e => e.Department == "HR");

Will return

var employee = new Employee
{
    Id = 2,
    Name = "Pinki",
    Department = "HR"
};

Example 3

var result = employeeList.First(e => e.Id == 8);

This will throw an error because the employee with ID 8 does not exist in the employee list.

FirstOrDefault()

  1. It returns the first element of a sequence, or a default value if no element is found.
  2. It throws an error only if the source is null.
  3. We should use it if more than one element is expected and you want only the first element. It's also good if the result is empty.

Example 1

var result = employeeList.FirstOrDefault();

Will return

new Employee() { Id = 1, Name = "Sunny", Department = "Technical" }

Example 2

var result = employeeList.FirstOrDefault(e => e.Department == "HR");

Will return

new Employee() { Id = 2, Name = "Pinki", Department = "HR" }

Example 3

var result = employeeList.FirstOrDefault(e => e.Id == 8);

Will not throw an error but it returns the default value of Employee.

Single()

  1. It returns the only item of a sequence.
  2. This will throw an exception if the result contains 0 or more than 1 elements.
  3. We should use it when we know that exactly one element is expected but neither 0 nor 2 or more.

Example 1

var result = employeeList.Single(e => e.Id == 1);

Will return

new Employee() { Id = 1, Name = "Sunny", Department = "Technical" }

Example 2

var result = employeeList.FirstOrDefault(e => e.Department == "HR");

Will throw an exception as 2 employees exist with the Department as "HR".

Example 3

var result = employeeList.Single(e => e.Id == 8);

Will throw an exception as no employee exists with Id as 8.

SingleOrDefault()

  1. It returns a single specific element, and if the element is not found, it returns the default value of it.
  2. This will throw an exception if the result contains 2 or more elements.
  3. We should use it when we know that 0 or 1 element is expected as a result.

Example 1

var result = employeeList.SingleOrDefault(e => e.Id == 1);

Will return

new Employee() { Id = 1, Name = "Sunny", Department = "Technical" }

Example 2

var result = employeeList.SingleOrDefault(e => e.Department == "HR");

Will throw an exception as 2 employees exist with the Department as HR employees.

Example 3

var result = employeeList.SingleOrDefault(e => e.Id == 8);

Will not throw an exception but it returns the default value of Employee.

I hope you understand the difference between First, FirstOrDefault and Single, SingleOrDefault.

Happy learning.
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