CompareTo()

Task of this program is merely to arrange the CarID in an ascending order. Please tell me what is the significant of the highlighted code. Because without highlighted code program is producing error signal.

using System;

namespace ObjComp
{
// The iteration of the Car can be ordered
// based on the CarID.
public class Car : IComparable
{
// State data.
private int CarID;
private string petName;

// properties.
public int ID
{
get { return CarID; }
set { CarID = value; }
}
public string PetName
{
get { return petName; }
set { petName = value; }
}

// Constructor set.
public Car(int id, string name)
{
this.CarID = id;
this.petName = name;
}

// IComparable implementation.
int IComparable.CompareTo(object o)
{
Car temp = (Car)o;
if (this.CarID > temp.CarID)
return 1;
if (this.CarID < temp.CarID)
return -1;
else
return 0;
}
}

public class CarApp
{
public static void Main(string[] args)
{
// Make an array of Car types.
Car[] myAutos = new Car[5];
myAutos[0] = new Car(123, "Rusty");
myAutos[1] = new Car(6, "Mary");
myAutos[2] = new Car(6, "Viper");
myAutos[3] = new Car(13, "NoName");
myAutos[4] = new Car(6, "Chucky");

// Now, sort them using IComparable.
Array.Sort(myAutos);

// Dump sorted array.
Console.WriteLine("***** Here is the ordered set of cars *****");
foreach (Car c in myAutos)
Console.WriteLine("{0} {1}", c.ID, c.PetName);

Console.Read();
}
}
}
/*
***** Here is the ordered set of cars *****
6 Chucky
6 Mary
6 Viper
13 NoName
123 Rusty
*/