IComparable Interface

Aug 26, 2008
by David Cumps

Part of my 2008 study notes for exam 70-536 (.NET Framework 2.0).

Overview

IComparable is required for sorting.
CompareTo is used for sorting, Equals is used to test for equality (IEquatable interface).
- Returns less than 0, 0 or greater than 0 to indicate how the current instance relates to the other object.

Good practice

For objects A, B and C, the following must be true:
A.CompareTo(A) is required to return zero.
If A.CompareTo(B) returns zero, then B.CompareTo(A) is required to return zero.
If A.CompareTo(B) returns zero and B.CompareTo(C) returns zero, then A.CompareTo(C) is required to return zero.
If A.CompareTo(B) returns a value other than zero, then B.CompareTo(A) is required to return a value of the opposite sign.
If A.CompareTo(B) returns a value x not equal to zero, and B.CompareTo(C) returns a value y of the same sign as x, then A.CompareTo(C) is required to return a value of the same sign as x and y.

Examples

Custom Sorting

struct MyPoint: IComparable
{
    private int _x;
    private int _y;
 
    public int X
    {
        get { return this._x; }
    }
 
    public int Y
    {
        get { return this._y; }
    }
 
    public MyPoint(int x, int y)
    {
        this._x = x;
        this._y = y;
    }
 
    #region IComparable Members
    public int CompareTo(object obj)
    {
        if (obj is MyPoint)
        {
            MyPoint other = (MyPoint)obj;
 
            if (this._x.CompareTo(other.X) == 0)
            {
                return this._y.CompareTo(other.Y);
            }
            else
            {
                return this._x.CompareTo(other.X);
            }
        }
        else
        {
            throw new NotSupportedException("obj is not a MyPoint");
        }
    }
    #endregion
}

List<MyPoint> list = new List<MyPoint>();
list.Add(new MyPoint(10, 5));
list.Add(new MyPoint(10, 2));
list.Add(new MyPoint(10, 3));
list.Add(new MyPoint(4, 1));
list.Add(new MyPoint(15, 1));
 
list.Sort();
 
foreach (MyPoint p in list)
{
    Console.WriteLine("({0}, {1})", p.X, p.Y);
}
 
/*
Will Print:
(4, 1)
(10, 2)
(10, 3)
(10, 5)
(15, 1)
*/