Homework - IComparable and IComparer - A step-by-step tutorial

1) Define your class City, as usual.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CityCompare
{
    public class City
    {
        //fields
        private string name;
        private string state;

        //properties
        public string Name
        {
            get { return name; }
            set { name = value; }
        }
        public string State
        {
            get { return state; }
            set { state = value; }
        }

        //constructors
        public City() { }   //null constructor
        public City(string _name, string _state)
        {
            this.name = _name;
            this.state = _state;
        }

        //override toString method
        public override string ToString()
        {
            return name + ", " + state;
        }
    }
}

2) We need IComparable to our class to provide for a simple sorting capability
We need to provide a simple sorting capability to the code. We make use of the interface IComparable. To do that, we need to tell the class to implement the interface IComparable. Just add a column (:) and the interface name next to the class name.

public class City : IComparable

3) IComparable Sorting capability - The CompareTo function
Now, we need to implement IComparable CompareTo function to provide the default sort order. We add the following code to our class:

//IComparable.CompareTo needs to be implemented 
        //to provide default sort order. Make it public
        public int CompareTo(Object obj)
        {
            City c = (City)obj; //cast Object type to City object
            return string.Compare(this.name, c.name);
        }

4) Writing your tester class with an ArrayList
In this tutorial, I used the Form1 class as the tester class. The tester class will populate the Arraylist with values.
A listbox is made available on the form so that the cities are listed there.
Also, a button that shows "Simple Sort" is available and uses the ArrayList.Sort() method and does not require any additional implementation.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Collections;       //add this line to use ArrayList


namespace CityCompare
{
    public partial class Form1 : Form
    {
        //fields
        private ArrayList cities = new ArrayList();

        //properties
        public ArrayList Cities
        {
            get { return cities; }
            set { cities = value; }
        }

        //constructors
        public Form1()
        {
            InitializeComponent();
        }

        //methods
        //Populate the listbox
        public void PopulateListBox()
        {
            //clear listbox
            lisCitiesList.Items.Clear();

            //Populate the listbox
            foreach (Object item in cities)
            {
                lisCitiesList.Items.Add(item.ToString());
            }
        }

        //Populate the ArrayList with cities and states
        public void PopulateArrayList()
        {
            cities.Add(new City("Sydney", "New South Wales"));
            cities.Add(new City("Albury", "New South Wales"));
            cities.Add(new City("Armindale", "New South Wales"));
            cities.Add(new City("Bathurst", "New South Wales"));
            cities.Add(new City("Blue Mountains", "New South Wales"));
            cities.Add(new City("Palmerston", "Northern Territory"));
            cities.Add(new City("Darwin", "Northern Territory"));
            cities.Add(new City("Melbourne", "Victoria"));
            cities.Add(new City("Perth", "Western Australia"));
            cities.Add(new City("Albany", "Western Australia"));
            cities.Add(new City("Canning", "Western Australia"));
            cities.Add(new City("Gosnells", "Western Australia"));
            cities.Add(new City("Hobart", "Tasmania"));
            cities.Add(new City("Hobart", "AnotherState"));
        }

        //events
        private void Form1_Load(object sender, EventArgs e)
        {
            //Populate cities arraylist
            PopulateArrayList();

            //Populate listbox
            PopulateListBox();
        }

        private void btnNormalSort_Click(object sender, EventArgs e)
        {
            cities.Sort();
            PopulateListBox();  //refresh list
        }

    }
}

5) Sort list of cities first by state name, then by city name, without modifying the implementation of the City class
As Alan stated, you cannot redefine the CompareTo method. Let's stick to the requirement and define a CityComparer class that implements the Comparator<City> interface.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;       //add this line to use interface IComparer

namespace CityCompare
{                                           //STEP 2
    public class CityComparer: IComparer    //we use this IComparer as it gives
                                            //additional comparison mechanisms
    {
        //we need to implement the interface member Compare(obj,obj)
        //from the interface IComparer
        public int Compare(Object a, Object b)  //make it public
        {
            City c1 = (City)a;  //cast Object to City
            City c2 = (City)b;  //cast Object to City

            return String.Compare(c1.State, c2.State);  //State is a string, 
                                                        //hence we use String.Compare
        }

        //StateSort method
        //sorts the ArrayList elements by state
        public static IComparer StateSort()
        {
            return (IComparer)new CityComparer();
        }
    }
}

6) Call the method StateSort to sort by state in the Form1 class.

private void btnCityComparer_Click(object sender, EventArgs e)
{
    cities.Sort(CityComparer.StateSort());  //STEP 3
    PopulateListBox();  //refresh list
}

All done! Suggestions and comments are welcome.