CSharp - Comparison between IComparer and IComparable

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Author: Jiangong SUN

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I've already written an article introducing the usage of comparer here. In this article I'll compare the usage of IComparable and IComparer with examples.

Important difference:  A class can have only one Comparable, but multiple Comparer.

Here I have a class Student with different properties, which will be used as sorting criterias.

        public class Student : IComparable<Student>        {            public string LastName { get; set; }            public string FirstName { get; set; }            public int Age { get; set; }            public int Class { get; set; }        }

Firstly, I want class Student to implement interface IComparable, and I must override method CompareTo of interface IComparable. In this method I sort Student list in an ascending order.

        //class who implement interface IComparable, must implement CompareTo method        public class Student : IComparable<Student>        {            public string LastName { get; set; }            public string FirstName { get; set; }            public int Age { get; set; }            public int Class { get; set; }            //CompareTo method compare itself to another object            public int CompareTo(Student student)            {                return LastName.CompareTo(student.LastName);            }        }

In this way, Student list will be sorted by LastName in an ascending order.

Next, I'll create 3 comparers who will use the rest 3 properties as sorting criteria.

        //Comparer 1        //class who implement interfce IComparer, must implement Compare method        public class OrderByAgeAscending : IComparer<Student>        {            //Compare method compares two objects            public int Compare(Student x, Student y)            {                if (x.Age.CompareTo(y.Age) < 0)                {                    return -1;                }                if (x.Age.CompareTo(y.Age) == 0)                {                    return 0;                }                return 1;            }        }        //Comparer 2        public class OrderByClassDescending : IComparer<Student>        {            public int Compare(Student x, Student y)            {                if (x.Class.CompareTo(y.Class) < 0)                {                    return 1;                }                if (x.Class.CompareTo(y.Class) == 0)                {                    return 0;                }                return -1;            }        }        //Comparer 3        public class OrderByFirstNameAscending : IComparer<Student>        {            public int Compare(Student x, Student y)            {                if (x.FirstName.CompareTo(y.FirstName) < 0)                {                    return -1;                } if (x.FirstName.CompareTo(y.FirstName) == 0)                {                    return 0;                }                return 1;            }        }

Now I will create a new Student list with some Students and try to sort them with comparable and comparer.

Firstly, I will create a list.

            var s1 = new Student() { LastName = "charles", FirstName = "charles", Age = 27, Class = 15 };            var s2 = new Student() { LastName = "viz", FirstName = "newton", Age = 20, Class = 30 };            var s3 = new Student() { LastName = "la", FirstName = "aba", Age = 2, Class = 2 };            var students = new List<Student>() { s1, s2, s3 };

Then, I will sort the list with comparable. I just need to call Sort() method, it will use the CompareTo() method I've created.

            //IComparable: sort by last name            students.Sort();            Display(students, "IComparable OrderByLastName:");

And then, I will use the 3 comparers to sort the list.

            //Comparer 1: order by age            var c1 = new OrderByAgeAscending();            students.Sort(c1);            Display(students, "Comparer OrderByAgeAscending:");            //Comparer 2: order by class            var c2 = new OrderByClassDescending();            students.Sort(c2);            Display(students, "Comparer OrderByClassDescending:");            //Comparer 3: order by first name             var c3 = new OrderByFirstNameAscending();            students.Sort(c3);            Display(students, "Comparer OrderByFirstNameAscending");

And there are an Display() method who is in charge of display the student information.

        public static void Display(List<Student> students, string comparerName)        {            Console.WriteLine(comparerName);            foreach (var student in students)            {                Console.WriteLine("last name: " + student.LastName + "; first name: " + student.FirstName + "; age: " + student.Age + "; class: " + student.Class);            }        }

Now, let's see the results:

Right now, we are arrived at the end of the article. I hope you can find useful information here. Enjoy coding!

references:

http://addinit.com/node/50

http://www.codeproject.com/Articles/42839/Sorting-Lists-using-IComparable-and-IComparer-Inte