LINQ ThenBy Operator

LINQ ThenBy Operator is used when we want to sort the elements in a collection by using multiple properties in ascending order.

This operator must use after OrderBy or OrderByDescending operator. First OrderBy operator sort the collection based on a single property and then we can use ThenBy operator to sort the collection by second property. We can use multiple ThenBy operator in a collection.

Below is the syntax of ThenBy operator.

public static IOrderedEnumerable<TSource> ThenBy<TSource, TKey>(this IOrderedEnumerable<TSource> source, Func<TSource, TKey> keySelector);

ThenBy is an extension method which takes single element and returns the property or field name.

C# ThenBy Example in Query Syntax

List<Student> students = new List<Student>();
students.Add(new Student { Id = 1, Name = "Ramesh", Rank = 1, Age = 39 });
students.Add(new Student { Id = 2, Name = "Kapil", Rank = 1, Age = 32 });
students.Add(new Student { Id = 3, Name = "Suresh", Rank = 2, Age = 45 });
students.Add(new Student { Id = 4, Name = "Mahesh", Rank = 2, Age = 39 });

var studentsOrderByRank = from student in students
                            orderby student.Rank, student.Age
                            select student;

Console.WriteLine("Sorted Students:");
foreach (var student in studentsOrderByRank)
{
    Console.WriteLine(student.Name);
}

Result
------
Sorted Students:
Kapil
Ramesh
Mahesh
Suresh

In the above example, we sort the collection based on Rank and Age properties. First collection is sorted based on Rank property and then by Age property. If we want to further sort the collection we can add another property by appending comma and property name.

C# ThenBy Example in Method Syntax

List<Student> students = new List<Student>();
students.Add(new Student { Id = 1, Name = "Ramesh", Rank = 1, Age = 39 });
students.Add(new Student { Id = 2, Name = "Kapil", Rank = 1, Age = 32 });
students.Add(new Student { Id = 3, Name = "Suresh", Rank = 2, Age = 45 });
students.Add(new Student { Id = 4, Name = "Mahesh", Rank = 2, Age = 39 });

var studentsOrderByRank = students.OrderBy(w => w.Rank).ThenBy(w => w.Age);

Console.WriteLine("Sorted Students:");
foreach (var student in studentsOrderByRank)
{
    Console.WriteLine(student.Name);
}

Result
------
Sorted Students:
Kapil
Ramesh
Mahesh
Suresh

We use method syntax of OrderBy and ThenBy operators to sort the collection based on first Rank property and then by Age property.