The C# libraries already implement the Quicksort sorting algorithm. So for either a string or an int, sorting the list is as straightforward as calling “Sort”, as show in the below example.

using System;
using System.Collections.Generic;

namespace IComparer
{
    class MainClass
    {
        public static void Main(string[] args)
        {

            List<string> myList = new List<string>();

            myList.Add("z");
            myList.Add("c");
            myList.Add("a");
            myList.Add("d");
            myList.Add("b");

            myList.Sort();

            int cnt = 0;
            foreach(string letter in myList)
                Console.WriteLine("{0} = {1}",cnt++,letter);
        }
    }
}

This code will output:

0 = a
1 = b
2 = c
3 = d
4 = z

But what to do if you would like to reverse the sorting order ? We will have to define our own reverse string comparison routine by sub-classing IComparer. In the essential Compare function we simply call the standard String.Compare function with the parameters in reverse order:

using System;
using System.Collections.Generic;

namespace IComparer
{

    public class ReverseStringComparer : IComparer<string>
    {
        public int Compare(string x, string y)
        {
            // We reverse the result by flipping the input parameters
            return String.Compare(y,x);
        }
    }

    class MainClass
    {
        public static void Main(string[] args)
        {

            List<string> myList = new List<string>();

            myList.Add("z");
            myList.Add("c");
            myList.Add("a");
            myList.Add("d");
            myList.Add("b");

            ReverseStringComparer myComparer = new ReverseStringComparer();        

            myList.Sort(myComparer);

            int cnt = 0;
            foreach(string letter in myList)
                Console.WriteLine("{0} = {1}",cnt++,letter);
        }
    }
}

This code will output:

0 = z
1 = d
2 = c
3 = b
4 = a

Both strings and numbers are very basic types. If you want to sort a list build out of your own datatypes you will need to provide your own ICompare derived comparison class. In the following example we we create struct contained a basic personal record with name, familyname and age. We provide two comparers, one that can sort the list by FamilyName and a second one that sorts the people in the list by their age.

using System;
using System.Collections.Generic;

namespace IComparer
{

    struct Person
    {
        public string FamilyName;
        public string Name;
        public int    Age;
        public Person(string iFamilyName, string iName, int iAge)
        {
            FamilyName = iFamilyName; Name = iName; Age = iAge;
        }
    }

    class PersonComparerByName : IComparer<Person>
    {
        public int Compare(Person x, Person y)
        {
            return String.Compare(x.FamilyName,y.FamilyName);
        }
    }

    class PersonComparerByAge : IComparer<Person>
    {
        public int Compare(Person x, Person y)
        {
            return x.Age - y.Age;
        }
    }

    class MainClass
    {
        public static void Main(string[] args)
        {

            List<Person> myList = new List<Person>();

            myList.Add( new Person("Mulder","Jan",22) );
            myList.Add( new Person("Dijkstra","Hans",45) );
            myList.Add( new Person("Zomer","Nara",25) );
            myList.Add( new Person("Italy","Jaro",38) );
            myList.Add( new Person("Robert","Nico",15) );

            // Sort the people in the list by their last name
            PersonComparerByName myComparerName = new PersonComparerByName();
            myList.Sort(myComparerName);

            int cnt1 = 0;
            foreach(Person aPerson in myList)
                Console.WriteLine("{0} = {1}",cnt1++,aPerson.FamilyName);

            // Sort the people in the list by their age
            PersonComparerByAge myComparerAge = new PersonComparerByAge();
            myList.Sort(myComparerAge);

            int cnt2 = 0;
            foreach(Person aPerson in myList)
                Console.WriteLine("{0} = {1} {2}",cnt2++,aPerson.FamilyName,aPerson.Age);

        }
    }
}

When run, the above code will output:

0 = Dijkstra
1 = Italy
2 = Mulder
3 = Robert
4 = Zomer

0 = Robert 15
1 = Mulder 22
2 = Zomer 25
3 = Italy 38
4 = Dijkstra 45

This is a post from Martijn's C# Coding Blog.