Implementing IComparable to Allow Sorting a Custom Type

Oct. 25, 2017

Introduction

One apple a day, keep doctor away. One program a day, also keep programmer awake. Today a small program keeps me awake to look into a few things, IComparer vs IComparable, throw new ArgumentExcecption on line 44. 

I like to share my practice using C# 2000 things, item 135: Implement IComparable to allow sorting a custom type.

Here is the C# code.

	using System;
	using System.Collections.Generic;
	using System.Linq;
	using System.Text;
	using System.Threading.Tasks;
	namespace ArraySort_Example
	{
	/// <summary>
	/// Oct. 25, 2017
	/// Julia learns C# 2000 things
	/// #135 – Implementing IComparable to Allow Sorting a Custom Type
	/// https://csharp.2000things.com/2010/10/30/135-implementing-icomparable-to-allow-sorting-a-custom-type/
	/// Arrays of elements that belong to a custom type cannot be sorted, unless the type implements
	/// the IComparable interface.
	///
	/// To make elements of a custom type sortable, you need to implement IComparable in your type.
	/// IComparable consists of the single method CompareTo, which compares two objects.
	///
	/// Here’s an example of a Person class implementing CompareTo to sort people in LastName/FirstName order:
	/// </summary>
	class Person : IComparable
	{
	public string FirstName { get; set; }
	public string LastName { get; set; }
	public Person(string firstName, string lastName)
	{
	FirstName = firstName;
	LastName = lastName;
	}
	/// <summary>
	/// CompareTo
	/// </summary>
	/// <param name="obj"></param>
	/// <returns></returns>
	public int CompareTo(object obj)
	{
	Person other = obj as Person;
	if (other == null)
	{
	throw new ArgumentException("Object is not a Person");
	}
	else
	{
	// Sort by LastName, then by FirstName (ignore case)
	int compare = this.LastName.ToLower().CompareTo(other.LastName.ToLower());
	if (compare == 0)
	{
	compare = this.FirstName.ToLower().CompareTo(other.FirstName.ToLower());
	}
	return compare;
	}
	}
	}
	class Program
	{
	static void Main(string[] args)
	{
	Person[] folks = new Person[4];
	folks[0] = new Person("Bronte", "Emily");
	folks[1] = new Person("Bronte", "Charlotte");
	folks[2] = new Person("Tennyson", "Alfred");
	folks[3] = new Person("Mailer", "Norman");
	Array.Sort(folks); // C. Bronte, E. Bronte, Mailer, Tennyson
	}
	}
	}

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