IEqualityComparer(T) Interface
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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


The type of objects to compare.

System_CAPS_pubmethodEquals(T, T)

Determines whether the specified objects are equal.


Returns 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();

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

      var redBox = new Box(4, 3, 4);
      AddBox(boxes, redBox, "red");

      var blueBox = new Box(4, 3, 4);
      AddBox(boxes, blueBox, "blue");

      var greenBox = new Box(3, 4, 3);
      AddBox(boxes, greenBox, "green");

      Console.WriteLine("The dictionary contains {0} Box objects.",

   private static void AddBox(Dictionary<Box, String> dict, Box box, String name)
      try {
         dict.Add(box, name);
      catch (ArgumentException e) {
         Console.WriteLine("Unable to add {0}: {1}", box, e.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; }

    public override String ToString()
       return String.Format("({0}, {1}, {2})", Height, Length, Width);

class BoxEqualityComparer : IEqualityComparer<Box>
    public bool Equals(Box b1, Box b2)
        if (b2 == null && b1 == null)
           return true;
        else if (b1 == null | b2 == null)
           return false;
        else 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();
// The example displays the following output:
//    Unable to add (4, 3, 4): An item with the same key has already been added.
//    The dictionary contains 2 Box objects.

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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