LINQ ThenBy

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 the source 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 the source 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 - $65,000.00Code 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 the OrderBy() method.