IComparable

This C# interface allows custom sorting of objects when implemented. When a class implements this interface, we must add the public method CompareTo.

Custom implementation

We implement custom sorting for a C# class with IComparable. We can sort in ascending and descending orders.

Example code

Examine the Employee class. This class implements IComparable for Employee, which means an Employee instance can be compared with other Employee instances.

using System;
using System.Collections.Generic;

class Employee : IComparable<Employee>
{
    public int Salary { get; set; }
    public string Name { get; set; }

    public int CompareTo(Employee other)
    {
        // Alphabetic sort if salary is equal. [A to Z]
        if (this.Salary == other.Salary)
        {
            return this.Name.CompareTo(other.Name);
        }
        // Default to salary sort. [High to low]
        return other.Salary.CompareTo(this.Salary);
    }

    public override string ToString()
    {
        // String representation.
        return this.Salary.ToString() + "," + this.Name;
    }
}

class Program
{
    static void Main()
    {
        List<Employee> list = new List<Employee>();
        list.Add(new Employee() { Name = "Steve", Salary = 10000 });
        list.Add(new Employee() { Name = "Janet", Salary = 10000 });
        list.Add(new Employee() { Name = "Andrew", Salary = 10000 });
        list.Add(new Employee() { Name = "Bill", Salary = 500000 });
        list.Add(new Employee() { Name = "Lucy", Salary = 8000 });

        // Uses IComparable.CompareTo()
        list.Sort();

        // Uses Employee.ToString
        foreach (var element in list)
        {
            Console.WriteLine(element);
        }
    }
}
500000,Bill
10000,Andrew
10000,Janet
10000,Steve
8000,Lucy

Reverse sorts

What should you do if you want to sort in reverse order in the CompareTo method? You could simply reverse the order of the CompareTo expression.

return other.Salary.CompareTo(this.Salary);
return this.Salary.CompareTo(other.Salary);

Alternatives

There are alternative ways you can sort collections of objects by two properties. The easiest is probably the orderby clause in the query syntax in System.Linq.

List syntax

You can specify the Comparison delegate in the lambda expression syntax. In this case, you do not need to implement the IComparable interface at all.

A summary

Through this example program, we implemented the IComparable interface on a class. The List.Sort method uses this interface to sort an object collection.