EqualityComparer<T>.Default 屬性
定義
重要
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傳回泛型引數指定之類型的預設相等比較子 (Comparer)。
public:
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)屬性值
型別 T 之 EqualityComparer<T> 類別的預設執行個體。
範例
下列範例會建立包含 型別元素的 Box 集合，然後藉由呼叫 方法兩次來搜尋符合另一個方塊的 FindFirst 方塊。
第一個搜尋不會指定任何相等比較子，這表示 FindFirst 使用 EqualityComparer<T>.Default 來判斷方塊的相等性。 接著會使用 類別中 方法的 IEquatable<T>.Equals Box 實作。 如果兩個方塊的維度相同，則會視為相等。
第二個搜尋會指定相等比較子 (BoxEqVolume) ，以依磁片區定義相等。 如果兩個方塊的磁片區相同，則會視為相等。
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.
備註
屬性 Default 會檢查類型 T 是否實作 System.IEquatable<T> 介面，如果是，則會傳回 EqualityComparer<T> 使用該實作的 。 否則，它會傳回 EqualityComparer<T> ，其會使用 的 Object.Equals 覆寫，並由 Object.GetHashCode 提供 T 。