EqualityComparer<T>.Default Property


返回一个默认的相等比较器,用于比较此泛型自变量指定的类型。Returns a default equality comparer for the type specified by the generic argument.

 static property System::Collections::Generic::EqualityComparer<T> ^ Default { System::Collections::Generic::EqualityComparer<T> ^ get(); };
public static System.Collections.Generic.EqualityComparer<T> Default { get; }
member this.Default : System.Collections.Generic.EqualityComparer<'T>
Public Shared ReadOnly Property Default As EqualityComparer(Of T)

Property Value


EqualityComparer<T> 类的类型为 T 的默认实例。The default instance of the EqualityComparer<T> class for type T.


下面的示例创建一个集合,该集合包含 Box 类型的元素,然后通过调用 FindFirst 方法两次,在该集合中搜索与其他框匹配的框。The following example creates a collection that contains elements of the Box type and then searches it for a box matching another box by calling the FindFirst method, twice.

第一次搜索未指定任何相等比较器,这意味着 FindFirst 使用 EqualityComparer<T>.Default 来确定相等的框。The first search does not specify any equality comparer, which means FindFirst uses EqualityComparer<T>.Default to determine equality of boxes. 这反过来会使用 Box 类中 IEquatable<T>.Equals 方法的实现。That in turn uses the implementation of the IEquatable<T>.Equals method in the Box class. 如果两个框的维度相同,则将它们视为相等。Two boxes are considered equal if their dimensions are the same.

第二个搜索指定的相等比较器(BoxEqVolume)定义按卷的相等性。The second search specifies an equality comparer (BoxEqVolume) that defines equality by volume. 如果两个框的卷相同,则将它们视为相等。Two boxes are considered equal if their volumes are the same.

using System;
using System.Collections.Generic;

static class Program
    static void Main()
        var redBox = new Box(8, 8, 4);
        var blueBox = new Box(6, 8, 4);
        var greenBox = new Box(4, 8, 8);

        var boxes = new[] { redBox, blueBox, greenBox };

        var boxToFind = new Box(4, 8, 8);

        var foundByDimension = boxes.FindFirst(boxToFind);

        Console.WriteLine($"Found box {foundByDimension} by dimension.");

        var foundByVolume = boxes.FindFirst(boxToFind, new BoxEqVolume());

        Console.WriteLine($"Found box {foundByVolume} by volume.");

public static class CollectionExtensions
    public static T FindFirst<T>(
        this IEnumerable<T> collection, T itemToFind, IEqualityComparer<T> comparer = null)
        comparer = comparer ?? EqualityComparer<T>.Default;

        foreach (var item in collection)
            if (comparer.Equals(item, itemToFind))
                return item;

        throw new InvalidOperationException("No matching item found.");

public class BoxEqVolume : EqualityComparer<Box>
    public override bool Equals(Box b1, Box b2)
        if (object.ReferenceEquals(b1, b2))
            return true;

        if (b1 is null || b2 is null)
            return false;

        return b1.Volume == b2.Volume;

    public override int GetHashCode(Box box) => box.Volume.GetHashCode();

public class Box : IEquatable<Box>
    public Box(int height, int length, int width)
        this.Height = height;
        this.Length = length;
        this.Width = width;

    public int Height { get; }
    public int Length { get; }
    public int Width { get; }

    public int Volume => Height * Length * Width;

    public bool Equals(Box other)
        if (other is null)
            return false;

        return this.Height == other.Height && this.Length == other.Length
            && this.Width == other.Width;

    public override bool Equals(object obj) => Equals(obj as Box);
    public override int GetHashCode() => (Height, Length, Width).GetHashCode();

    public override string ToString() => $"{Height} x {Length} x {Width}";

/* This example produces the following output:
    Found box 4 x 8 x 8 by dimension.
    Found box 8 x 8 x 4 by volume.
Imports System.Collections.Generic
Imports System.Runtime.CompilerServices

Public Class Example
    Public Shared Sub Main()
        Dim redBox As New Box(8, 8, 4)
        Dim blueBox As New Box(6, 8, 4)
        Dim greenBox As New Box(4, 8, 8)

        Dim boxes As Box() = { redBox, blueBox, greenBox }

        Dim boxToFind As New Box(4, 8, 8)

        Dim foundByDimension = boxes.FindFirst(boxToFind)

        Console.WriteLine($"Found box {foundByDimension} by dimension.")

        Dim foundByVolume = boxes.FindFirst(boxToFind, New BoxEqVolume())

        Console.WriteLine($"Found box {foundByVolume} by volume.")
    End Sub

    Private Shared Sub PrintBoxCollection(boxes As Dictionary(Of Box, String))
        For Each kvp As KeyValuePair(Of Box, String) In boxes
            Console.WriteLine($"{kvp.Key.Height} x {kvp.Key.Length} x {kvp.Key.Width} - {kvp.Value}")
    End Sub
End Class

Public Module CollectionExtensions
    Public Function FindFirst(Of T)(
        collection As IEnumerable(Of T), itemToFind As T, Optional comparer As IEqualityComparer(Of T) = Nothing)

        comparer = If(comparer, EqualityComparer(Of T).Default)

        For Each item In collection
            If comparer.Equals(item, itemToFind)
                Return item
            End IF

        Throw New InvalidOperationException("No matching item found.")
    End Function
End Module

Public Class BoxEqVolume
    Inherits EqualityComparer(Of Box)

    Public Overrides Function GetHashCode(box As Box) As Integer
        Return box.Volume.GetHashCode()
    End Function

    Public Overrides Function Equals(b1 As Box, b2 As Box) As Boolean
        If b1 Is b2 Then
            Return True
        End If

        If b1 Is Nothing OrElse b2 Is Nothing Then
            Return False
        End If

        Return b1.Volume = b2.Volume
    End Function
End Class

Public Class Box
    Implements IEquatable(Of Box)

    Public Sub New(height As Integer, length As Integer, width As Integer)
        Me.Height = height
        Me.Length = length
        Me.Width = width
    End Sub

    Public ReadOnly Property Height() As Integer
    Public ReadOnly Property Length() As Integer
    Public ReadOnly Property Width() As Integer

    Public ReadOnly Property Volume() As Integer
            Return Height * Length * Width
        End Get
    End Property

    Public Overloads Function Equals(other As Box) As Boolean Implements IEquatable(Of Box).Equals
        If other Is Nothing Then
            Return False
        End If

        Return Me.Height = other.Height AndAlso Me.Length = other.Length AndAlso
            Me.Width = other.Width
    End Function

    Public Overrides Function Equals(other As Object) As Boolean
        Return Equals(TryCast(other, Box))
    End Function

    Public Overrides Function GetHashCode() As Integer
        Return (Height, Length, Width).GetHashCode()
    End Function

    Public Overrides Function ToString() As String
        Return $"{Height} x {Length} x {Width}"
    End Function
End Class
' This example produces the following output:
' Found box 4 x 8 x 8 by dimension.
' Found box 8 x 8 x 4 by volume.


Default 属性检查类型 T 是否实现了 System.IEquatable<T> 接口,如果是,则返回使用该实现的 EqualityComparer<T>The Default property checks whether type T implements the System.IEquatable<T> interface and, if so, returns an EqualityComparer<T> that uses that implementation. 否则,它将返回 EqualityComparer<T>,它使用 T提供的 Object.EqualsObject.GetHashCode 的重写。Otherwise, it returns an EqualityComparer<T> that uses the overrides of Object.Equals and Object.GetHashCode provided by T.

Applies to

See also