EqualityComparer<T>.Default 属性

定义

命名空间:

System.Collections.Generic

程序集:

System.Collections.dll

Source:

EqualityComparer.CoreCLR.cs

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。

public:
 static property System::Collections::Generic::EqualityComparer<T> ^ Default { System::Collections::Generic::EqualityComparer<T> ^ get(); };

public static System.Collections.Generic.EqualityComparer<T> Default { get; }

static member Default : System.Collections.Generic.EqualityComparer<'T>

Public Shared ReadOnly Property Default As EqualityComparer(Of T)

属性值

EqualityComparer<T>

EqualityComparer<T> 类的类型为 T 的默认实例。

示例

以下示例创建一个包含 类型元素的 Box 集合，然后通过调用 方法两次来搜索与另一个框匹配的 FindFirst 框。

第一个搜索未指定任何相等比较器，这意味着 FindFirst 使用 EqualityComparer<T>.Default 来确定框的相等性。 这反过来又使用 类中 Box 方法的IEquatable<T>.Equals实现。 如果两个框的尺寸相同，则视为相等。

第二个搜索指定 () 相等比较器，该比较器 BoxEqVolume 按卷定义相等性。 如果两个框的卷相同，则视为相等。

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}")
        Next
    End Sub
End Class

Public Module CollectionExtensions
    <Extension()> 
    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
        Next

        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
        Get
            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.

注解

属性 Default 检查类型 T 是否实现 System.IEquatable<T> 接口，如果是，则 EqualityComparer<T> 返回使用该实现的 。 否则，它将返回一个 EqualityComparer<T> ，它使用 的 替代， Object.Equals 并由 Object.GetHashCodeT提供。

另请参阅

• IEqualityComparer<T>
• IEquatable<T>
• Object