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IEqualityComparer<T> Interface

Updated: September 2009

Defines methods to support the comparison of objects for equality.

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

public interface IEqualityComparer<T>

Type Parameters

T

The type of objects to compare.

This interface allows the implementation of customized equality comparison for collections. That is, you can create your own definition of equality for type T, and specify that this definition be used with a collection type that accepts the IEqualityComparer<T> generic interface. In the .NET Framework, constructors of the Dictionary<TKey, TValue> generic collection type accept this interface.

A default implementation of this interface is provided by the Default property of the EqualityComparer<T> generic class. The StringComparer class implements IEqualityComparer<T> of type String.

This interface supports only equality comparisons. Customization of comparisons for sorting and ordering is provided by the IComparer<T> generic interface.

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 adds custom Box objects to a dictionary collection. The Box objects are considered equal if their dimensions are the same.

using System;
using System.Collections.Generic;
class Example
{
    static void Main()
    {
        try
        {

            BoxEqualityComparer boxEqC = new BoxEqualityComparer();

            Dictionary<Box, String> boxes = new Dictionary<Box,
                                                string>(boxEqC);

            Box redBox = new Box(4, 3, 4);
            Box blueBox = new Box(4, 3, 4);

            boxes.Add(redBox, "red");
            boxes.Add(blueBox, "blue");

        }
        catch (ArgumentException argEx)
        {

            Console.WriteLine(argEx.Message);
        }
    }
}

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 BoxEqualityComparer : IEqualityComparer<Box>
{

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

}

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

September 2009

Added example and revised remarks.

Information enhancement.

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