IComparable & IComparer

Syntax

class X : IComparable<X> { public int CompareTo(X other) { } }

Examples

public class Student : IComparable<Student>
{
    public string Name;
    public int Grade;

    public Student(string name, int grade) { Name = name; Grade = grade; }

    public int CompareTo(Student? other) => Grade.CompareTo(other?.Grade);

    public override string ToString() => $"{Name}({Grade})";
}

List<Student> students = new List<Student>
{
    new Student("Fola", 85),
    new Student("Zain", 92),
    new Student("Jamal", 78)
};

students.Sort();  // uses CompareTo() automatically
Console.WriteLine(string.Join(", ", students));  // Jamal(78), Fola(85), Zain(92)

public class ByNameComparer : IComparer<Student>
{
    public int Compare(Student? x, Student? y) => string.Compare(x?.Name, y?.Name);
}

List<Student> students = new List<Student>
{
    new Student("Zain", 92),
    new Student("Fola", 85)
};

students.Sort(new ByNameComparer());
Console.WriteLine(string.Join(", ", students));  // Fola(85), Zain(92)

List<Student> students = new List<Student>
{
    new Student("Zain", 92),
    new Student("Fola", 85),
    new Student("Jamal", 78)
};

students.Sort((a, b) => b.Grade.CompareTo(a.Grade));  // descending by grade
Console.WriteLine(string.Join(", ", students));  // Zain(92), Fola(85), Jamal(78)

Best practices

• Implement IComparable<T> when a class has one obvious, natural ordering (like numbers by value, or dates chronologically)
• Write a separate IComparer<T> class (or use a lambda) when you need multiple different orderings, or cannot modify the class being sorted
• Use a lambda passed to Sort()/OrderBy() for simple, one-off custom orderings instead of writing a full IComparer<T> class
• Handle null carefully in CompareTo()/Compare() implementations - a naive call can throw a NullReferenceException on unexpected null input

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation