Summary: in this tutorial, you’ll learn how to use the LINQ ThenBy()
method to sort a sequence in ascending order based on a secondary key.
Introduction to the LINQ ThenBy() method
The LINQ ThenBy()
method sorts a sequence in ascending order based on a secondary key after the sequence has been sorted by a primary key using the OrderBy()
or OrderByDescending()
method:
public static IOrderedEnumerable<TSource> ThenBy<TSource, TKey> (
this IOrderedEnumerable<TSource> source,
Func<TSource, TKey> keySelector
);
Code language: C# (cs)
In this syntax:
TSource
is the type of elements in thesource
sequence.TKey
is the type of sort key.source
is the input sequence to sort.keySelector
is a function that selects a key to sort thesource
sequence.
The ThenBy()
method returns a new sequence sorted by a key in ascending order with the type of IOrderedEnumerable<TSource>
.
If the source
or keySelector
is null, the method ThenBy()
method will throw an ArgumentNullException
.
LINQ ThenBy() method example
The following example use the OrderBy()
to sort a list of employees by department alphabetically first and then use the ThenBy()
method to sort the employees in each department by Salary
from low to high:
using static System.Console;
class Employee
{
public string? Name { get; set; }
public string? Department { get; set; }
public int Salary { get; set; }
}
class Program
{
public static void Main()
{
var employees = new List<Employee>()
{
new Employee { Name = "John", Department = "HR", Salary = 50000 },
new Employee { Name = "Jane", Department = "IT", Salary = 60000 },
new Employee { Name = "Bob", Department = "HR", Salary = 45000 },
new Employee { Name = "Sara", Department = "IT", Salary = 55000 },
new Employee { Name = "Tom", Department = "IT", Salary = 65000 }
};
var results = employees.OrderBy(e => e.Department).ThenBy(e => e.Salary);
foreach (var e in results)
{
WriteLine($"{e.Department} - {e.Name} - {e.Salary:C}");
}
}
}
Code language: C# (cs)
Output:
HR - Bob - $45,000.00
HR - John - $50,000.00
IT - Sara - $55,000.00
IT - Jane - $60,000.00
IT - Tom - $65,000.00
Code language: plaintext (plaintext)
How it works.
First, define the Employee
class that has three properties: Name
, Department
, and Salary
:
class Employee
{
public string? Name { get; set; }
public string? Department { get; set; }
public int Salary { get; set; }
}
Code language: C# (cs)
Second, create a new list that has five Employee
objects:
var employees = new List<Employee>()
{
new Employee { Name = "John", Department = "HR", Salary = 50000 },
new Employee { Name = "Jane", Department = "IT", Salary = 60000 },
new Employee { Name = "Bob", Department = "HR", Salary = 45000 },
new Employee { Name = "Sara", Department = "IT", Salary = 55000 },
new Employee { Name = "Tom", Department = "IT", Salary = 65000 }
};
Code language: C# (cs)
Third, sort the Employee
objects by departments in ascending order using the OrderBy()
method and then sort the Employee
objects in each department by salary using the ThenBy()
method:
var results = employees.OrderBy(e => e.Department)
.ThenBy(e => e.Salary);
Code language: C# (cs)
Finally, write the employees to the console using a foreach
and WriteLine()
method:
foreach (var e in results)
{
WriteLine($"{e.Department} - {e.Name} - {e.Salary:C}");
}
Code language: C# (cs)
Summary
- Use LINQ
ThenBy()
method to sort elements in a sequence in ascending order by a secondary key, after the sequence has been sorted by a primary key in ascending order using theOrderBy()
method.