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IEqualityComparer<T> Interface

Defines methods to support the comparison of objects for equality.

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

public interface IEqualityComparer<in T>

Type Parameters

in T

The type of objects to compare.

This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.

The IEqualityComparer<T> type exposes the following members.

Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryEqualsDetermines whether the specified objects are equal.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetHashCodeReturns a hash code for the specified object.

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()

            BoxEqualityComparer boxEqC = new BoxEqualityComparer();

            Dictionary<Box, String> boxes = new Dictionary<Box,

            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)


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;
            return false;

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


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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