EqualityComparer<T>.Default EqualityComparer<T>.Default EqualityComparer<T>.Default EqualityComparer<T>.Default Property


傳回泛型引數指定之型別的預設相等比較子 (Comparer)。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)


型別 TEqualityComparer<T> 類別的預設執行個體。The default instance of the EqualityComparer<T> class for type T.


下列範例會建立集合,其中包含的項目Box輸入,然後將它搜尋方塊,藉由呼叫符合另一個方塊FindFirst方法兩次。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.

第一次搜尋未指定任何的相等比較子,這表示FindFirst使用EqualityComparer<T>.Default來判斷是否相等的方塊。The first search does not specify any equality comparer, which means FindFirst uses EqualityComparer<T>.Default to determine equality of boxes. 依序使用的實作IEquatable<T>.Equals方法中的Box類別。That in turn uses the implementation of the IEquatable<T>.Equals method in the Box class. 這兩個箱子會被視為相等,如果維度相同的。Two boxes are considered equal if their dimensions are the same.

第二個搜尋指定的相等比較子 (BoxEqVolume) 的磁碟區所定義等號比較。The second search specifies an equality comparer (BoxEqVolume) that defines equality by volume. 這兩個箱子會被視為相等,如果其磁碟區相同。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屬性檢查是否輸入T實作System.IEquatable<T>介面,並因此,如果傳回EqualityComparer<T>使用該實作。The Default property checks whether type T implements the System.IEquatable<T> interface and, if so, returns an EqualityComparer<T> that uses that implementation. 否則,它會傳回EqualityComparer<T>使用的覆寫Object.EqualsObject.GetHashCode所提供TOtherwise, it returns an EqualityComparer<T> that uses the overrides of Object.Equals and Object.GetHashCode provided by T.