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C# IComparable tutorial

C# IComparable tutorial shows how to compare values in C# with IComparable interface. C# tutorial is a comprehensive tutorial on C# language.

C# IComparable interface

The IComparable interface defines a generalized type-specific comparison method that a value type or class implements to order or sort its instances.

The IComparable is implemented by types whose values can be ordered or sorted. The interface requires the CoompareTo() method to be implemented. The implemented method is automatically called by methods such as Array.Sort() and ArrayList.Sort().

The interface is used by types that the programmer has control over; in other words by code that he has written. If the code is provided by someone else, we use the IComparer interface instead.

C# IComparable example

In the following example, we sort a list of employees.

Program.cs
using System;
using System.Collections.Generic;

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

        public int CompareTo(Employee other)
        {
            if (this.Salary < other.Salary)
            {
                return 1;
            }
            else if (this.Salary > other.Salary)
            {
                return -1;
            }
            else
            {
                return 0;
            }
        }

        public override string ToString()
        {

            return $"{this.Name} {this.Salary}";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var employees = new List<Employee>();
            employees.Add(new Employee() { Name = "John Doe", Salary = 1230 });
            employees.Add(new Employee() { Name = "Lucy Novak", Salary = 670 });
            employees.Add(new Employee() { Name = "Robin Brown", Salary = 2300 });
            employees.Add(new Employee() { Name = "Joe Draker", Salary = 1190 });
            employees.Add(new Employee() { Name = "Janet Does", Salary = 980 });

            employees.Sort();

            employees.ForEach(employee => Console.WriteLine(employee));
        }
    }
}

In the example, we provide an implementation of the CompareTo() method of the Employee class. The class implements the IComparable interface.

public int CompareTo(Employee other)
{
    if (this.Salary < other.Salary)
    {
        return 1;
    }
    else if (this.Salary > other.Salary)
    {
        return -1;
    }
    else
    {
        return 0;
    }
}

The implementation of the CompareTo() method will sort the employees by their salary.

employees.Sort();

We sort the list. The method takes the implemented CompareTo() method into account when sorting.

$ dotnet run
Robin Brown 2300
John Doe 1230
Joe Draker 1190
Janet Does 980
Lucy Novak 670

This is the output.

C# IComparable example II

In the following example, we sort an array of users.

Program.cs
using System;

namespace CompareUsersEx
{
    class User : IComparable<User>
    {
        public string Name { get; set; }
        public string Occupation { get; set; }

        public int CompareTo(User other)
        {
            if (this.Occupation == other.Occupation)
            {
                return this.Occupation.CompareTo(other.Occupation);
            }

            return this.Occupation.CompareTo(other.Occupation);
        }

        public override string ToString()
        {
            return $"{this.Name}, {this.Occupation.ToString()}";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var users = new User[] {
                new User() { Name = "Robin", Occupation = "bookseller" },
                new User() { Name = "John", Occupation = "gardener" },
                new User() { Name = "John", Occupation = "writer" },
                new User() { Name = "Janet", Occupation = "teacher" },
                new User() { Name = "Andrew", Occupation = "driver" },
                new User() { Name = "Lucy", Occupation = "accountant" }
            };

            Array.Sort(users);

            foreach (var user in users)
            {
                Console.WriteLine(user);
            }

            Console.WriteLine("********************************");

            Array.Reverse(users);

            foreach (var user in users)
            {
                Console.WriteLine(user);
            }
        }
    }
}

The example sorts an array of users in ascending and descending order.

public int CompareTo(User other)
{
    if (this.Occupation == other.Occupation)
    {
        return this.Occupation.CompareTo(other.Occupation);
    }

    return this.Occupation.CompareTo(other.Occupation);
}

The method sorts by the occupation.

Array.Sort(users);

We sort the users in ascending order.

Array.Reverse(users);

We sort the users in descending order.

$ dotnet run
Lucy, accountant
Robin, bookseller
Andrew, driver
John, gardener
Janet, teacher
John, writer
********************************
John, writer
Janet, teacher
John, gardener
Andrew, driver
Robin, bookseller
Lucy, accountant

This is the output.

In this tutorial, we have used IComparable interface to sort data in C#.

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