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EqualityComparer<T> Class

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

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

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.

The EqualityComparer<T> type exposes the following members.

  NameDescription
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsEqualityComparer<T>Initializes a new instance of the EqualityComparer<T> class.
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  NameDescription
Public propertyStatic memberSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsDefaultReturns a default equality comparer for the type specified by the generic argument.
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  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsEquals(T, T)When overridden in a derived class, determines whether two objects of type T are equal.
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetHashCode()Serves as the default hash function. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetHashCode(T)When overridden in a derived class, serves as a hash function for the specified object for hashing algorithms and data structures, such as a hash table.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsToStringReturns a string that represents the current object. (Inherited from Object.)
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  NameDescription
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsIEqualityComparer.EqualsDetermines whether the specified objects are equal.
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsIEqualityComparer.GetHashCodeReturns a hash code for the specified object.
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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 invokes the implementation of the IEquatable<T>.Equals 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.
 * 
*/

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 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

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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