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EqualityComparer<T> Class

Updated: January 2010

Provides a base class for implementations of the IEqualityComparer<T> generic interface.

Namespace:  System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)

[SerializableAttribute]
public abstract class EqualityComparer<T> : IEqualityComparer, 
	IEqualityComparer<T>

Type Parameters

T

The type of objects to compare.

Derive from this class to provide a custom implementation of the IEqualityComparer<T> generic interface for use with collection classes such as the Dictionary<TKey, TValue> generic class, or with methods such as List<T>.Sort.

The Default property checks whether type T implements the System.IEquatable<T> generic interface and, if so, returns an EqualityComparer<T> that contains the implantation of the IEquitableEquals method. Otherwise, it returns an EqualityComparer<T>, as provided by T.

We recommend that you derive from the EqualityComparer<T> class instead of implementing the IEqualityComparer<T> interface, because the EqualityComparer<T> class tests for equality using the IEquatable<T>.Equals method instead of the Object.Equals method. This is consistent with the Contains, IndexOf, LastIndexOf, and Remove methods of the Dictionary<TKey, TValue> class and other generic collections.

The following example creates a dictionary collection of objects of type Box with an equality comparer. Two boxes are considered equal if their dimensions are the same. It then adds the boxes to the collection.

The dictionary is recreated with an equality comparer that defines equality in a different way: Two boxes are considered equal if their volumes are the same.

using System;
using System.Collections.Generic;

class Program
{
	static Dictionary<Box, String> boxes;

	static void Main(string[] args)
	{

		BoxSameDimensions boxDim = new BoxSameDimensions();
		boxes = new Dictionary<Box, string>(boxDim);

		Console.WriteLine("Boxes equality by dimensions:");
		Box redBox = new Box(8, 4, 8);
		Box greenBox = new Box(8, 6, 8);
		Box blueBox = new Box(8, 4, 8);
		Box yellowBox = new Box(8, 8, 8); 
		AddBox(redBox, "red");
		AddBox(greenBox, "green");
		AddBox(blueBox, "blue");
		AddBox(yellowBox, "yellow");

		Console.WriteLine();
		Console.WriteLine("Boxes equality by volume:");

		BoxSameVolume boxVolume = new BoxSameVolume();
		boxes = new Dictionary<Box, string>(boxVolume);
		Box pinkBox = new Box(8, 4, 8);
		Box orangeBox = new Box(8, 6, 8);
		Box purpleBox = new Box(4, 8, 8);
		Box brownBox = new Box(8, 8, 4);
		AddBox(pinkBox, "pink");
		AddBox(orangeBox, "orange");
		AddBox(purpleBox, "purple");
		AddBox(brownBox, "brown");
	}

	public static void AddBox(Box bx, string name)
	{
		try
		{
			boxes.Add(bx, name);
			Console.WriteLine("Added {0}, Count = {1}, HashCode = {2}", 
				name, boxes.Count.ToString(), bx.GetHashCode());
		}
		catch (ArgumentException)
		{
			Console.WriteLine("A box equal to {0} is already in the collection.", name);
		}




	}
}
public class Box
{
	public Box(int h, int l, int w)
	{
		this.Height = h;
		this.Length = l;
		this.Width = w;
	}
	public int Height { get; set; }
	public int Length { get; set; }
	public int Width { get; set; }
}


class BoxSameDimensions : EqualityComparer<Box>
{

	public override bool Equals(Box b1, Box b2)
	{
		if (b1.Height == b2.Height & b1.Length == b2.Length
							& b1.Width == b2.Width)
		{
			return true;
		}
		else
		{
			return false;
		}
	}


	public override int GetHashCode(Box bx)
	{
		int hCode = bx.Height ^ bx.Length ^ bx.Width;
		return hCode.GetHashCode();
	}

}

class BoxSameVolume : EqualityComparer<Box>
{

	public override bool Equals(Box b1, Box b2)
	{
		if (b1.Height * b1.Width * b1.Length == 
			b2.Height * b2.Width * b2.Length)
		{
			return true;
		}
		else
		{
			return false;
		}
	}


	public override int GetHashCode(Box bx)
	{
		int hCode = bx.Height ^ bx.Length ^ bx.Width;
		return hCode.GetHashCode();
	}

}
/* This example produces the following output:
 * 
	Boxes equality by dimensions:
	Added red, Count = 1, HashCode = 46104728
	Added green, Count = 2, HashCode = 12289376
	A box equal to blue is already in the collection.
	Added yellow, Count = 3, HashCode = 55530882

	Boxes equality by volume:
	Added pink, Count = 1, HashCode = 30015890
	Added orange, Count = 2, HashCode = 1707556
	A box equal to purple is already in the collection.
	A box equal to brown is already in the collection.
 * 
*/

System.Object
  System.Collections.Generic.EqualityComparer<T>

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

January 2010

Updated example.

Information enhancement.

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