EqualityComparer<T>.Default Propriedade

Definição

Namespace:

System.Collections.Generic

Assembly:

System.Collections.dll

Origem:

EqualityComparer.CoreCLR.cs

Retorna um comparador de igualdade padrão para o tipo especificado pelo argumento genérico.

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)

Valor da propriedade

EqualityComparer<T>

A instância padrão da classe EqualityComparer<T> para o tipo T.

Exemplos

O exemplo a seguir cria uma coleção que contém elementos do Box tipo e, em seguida, pesquisa uma caixa correspondente a outra caixa chamando o FindFirst método, duas vezes.

A primeira pesquisa não especifica nenhum comparador de igualdade, o que significa FindFirst que usa EqualityComparer<T>.Default para determinar a igualdade de caixas. Isso, por sua vez, usa a implementação do IEquatable<T>.Equals método na Box classe . Duas caixas são consideradas iguais se suas dimensões forem as mesmas.

A segunda pesquisa especifica um comparador de igualdade (BoxEqVolume) que define a igualdade por volume. Duas caixas são consideradas iguais se seus volumes forem iguais.

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.

Comentários

A Default propriedade verifica se o tipo T implementa a System.IEquatable<T> interface e, nesse caso, retorna um EqualityComparer<T> que usa essa implementação. Caso contrário, ele retorna um EqualityComparer<T> que usa as substituições de Object.Equals e Object.GetHashCode fornecidos por T.

Confira também

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