集合内的比较和排序Comparisons and Sorts Within Collections

System.Collections 类在管理集合所涉及的几乎所有进程中执行比较,无论是搜索待删除的元素或返回键值对的值。The System.Collections classes perform comparisons in almost all the processes involved in managing collections, whether searching for the element to remove or returning the value of a key-and-value pair.

集合通常使用相等比较器和/或排序比较器。Collections typically utilize an equality comparer and/or an ordering comparer. 有两个构造用于比较。Two constructs are used for comparisons.

检查等同性Checking for equality

诸如 ContainsIndexOfLastIndexOfRemove 的方法将相等比较器用于集合元素。Methods such as Contains, IndexOf, LastIndexOf, and Remove use an equality comparer for the collection elements. 如果集合是泛型的,则按照以下原则比较项是否相等:If the collection is generic, than items are compared for equality according to the following guidelines:

此外,字典集合的某些构造函数重载接受 IEqualityComparer<T> 实现,用于比较键是否相等。In addition, some constructor overloads for dictionary collections accept an IEqualityComparer<T> implementation, which is used to compare keys for equality. 有关示例,请参见 Dictionary<TKey,TValue>.Dictionary<TKey,TValue> 构造函数。For an example, see the Dictionary<TKey,TValue>.Dictionary<TKey,TValue> constructor.

确定排序顺序Determining sort order

BinarySearchSort 等方法将排序比较器用于集合元素。Methods such as BinarySearch and Sort use an ordering comparer for the collection elements. 可在集合的元素间进行比较,或在元素或指定值之间进行比较。The comparisons can be between elements of the collection, or between an element and a specified value. 对于比较对象,有 default comparerexplicit comparer的概念。For comparing objects, there is the concept of a default comparer and an explicit comparer.

默认比较器依赖至少一个正在被比较的对象来实现 IComparable 接口。The default comparer relies on at least one of the objects being compared to implement the IComparable interface. 在用作列表集合的值或字典集合的键的所有类上实现 IComparable 不失为一个好办法。It is a good practice to implement IComparable on all classes are used as values in a list collection or as keys in a dictionary collection. 对泛型集合而言,等同性比较是根据以下内容确定的:For a generic collection, equality comparison is determined according to the following:

为了提供显式比较,某些方法接受 IComparer 实现作为参数。To provide explicit comparisons, some methods accept an IComparer implementation as a parameter. 例如, List<T>.Sort 方法接受 System.Collections.Generic.IComparer<T> 实现。For example, the List<T>.Sort method accepts an System.Collections.Generic.IComparer<T> implementation.

系统当前的区域性设置可影响集合中的比较和排序。The current culture setting of the system can affect the comparisons and sorts within a collection. 默认情况下, Collections 类中的比较和排序是区分区域性的。By default, the comparisons and sorts in the Collections classes are culture-sensitive. 若要忽略区域性设置并因此获得一致的比较和排序结果,请使用具有接受 InvariantCulture 的成员重载的 CultureInfoTo ignore the culture setting and therefore obtain consistent comparison and sorting results, use the InvariantCulture with member overloads that accept a CultureInfo. 有关详细信息,请参阅 Performing Culture-Insensitive String Operations in CollectionsPerforming Culture-Insensitive String Operations in ArraysFor more information, see Performing Culture-Insensitive String Operations in Collections and Performing Culture-Insensitive String Operations in Arrays.

等同性和排序示例Equality and sort example

以下代码展示了 IEquatable<T>IComparable<T> 在简单的业务对象上的实现。The following code demonstrates an implementation of IEquatable<T> and IComparable<T> on a simple business object. 此外,如果对象被存储在列表中并已排序,那么你会发现调用 Sort() 方法会导致 Part 类型使用默认比较器,并通过使用匿名方法实现 Sort(Comparison<T>) 方法。In addition, when the object is stored in a list and sorted, you will see that calling the Sort() method results in the use of the default comparer for the Part type, and the Sort(Comparison<T>) method implemented by using an anonymous method.

using System;
using System.Collections.Generic;
// Simple business object. A PartId is used to identify the type of part 
// but the part name can change. 
public class Part : IEquatable<Part> , IComparable<Part>
{
    public string PartName { get; set; }

    public int PartId { get; set; }

    public override string ToString()
    {
        return "ID: " + PartId + "   Name: " + PartName;
    }
    public override bool Equals(object obj)
    {
        if (obj == null) return false;
        Part objAsPart = obj as Part;
        if (objAsPart == null) return false;
        else return Equals(objAsPart);
    }
    public int SortByNameAscending(string name1, string name2)
    {
        
        return name1.CompareTo(name2);
    }

    // Default comparer for Part type.
    public int CompareTo(Part comparePart)
    {
          // A null value means that this object is greater.
        if (comparePart == null)
            return 1;
            
        else
            return this.PartId.CompareTo(comparePart.PartId);
    }
    public override int GetHashCode()
    {
        return PartId;
    }
    public bool Equals(Part other)
    {
        if (other == null) return false;
        return (this.PartId.Equals(other.PartId));
    }
    // Should also override == and != operators.

}
public class Example
{
    public static void Main()
    {
        // Create a list of parts.
        List<Part> parts = new List<Part>();

        // Add parts to the list.
        parts.Add(new Part() { PartName = "regular seat", PartId = 1434 });
        parts.Add(new Part() { PartName= "crank arm", PartId = 1234 });
        parts.Add(new Part() { PartName = "shift lever", PartId = 1634 }); ;
        // Name intentionally left null.
        parts.Add(new Part() {  PartId = 1334 });
        parts.Add(new Part() { PartName = "banana seat", PartId = 1444 });
        parts.Add(new Part() { PartName = "cassette", PartId = 1534 });
       

        // Write out the parts in the list. This will call the overridden 
        // ToString method in the Part class.
        Console.WriteLine("\nBefore sort:");
        foreach (Part aPart in parts)
        {
            Console.WriteLine(aPart);
        }


        // Call Sort on the list. This will use the 
        // default comparer, which is the Compare method 
        // implemented on Part.
        parts.Sort();


        Console.WriteLine("\nAfter sort by part number:");
        foreach (Part aPart in parts)
        {
            Console.WriteLine(aPart);
        }
       
        // This shows calling the Sort(Comparison(T) overload using 
        // an anonymous method for the Comparison delegate. 
        // This method treats null as the lesser of two values.
        parts.Sort(delegate(Part x, Part y)
        {
            if (x.PartName == null && y.PartName == null) return 0;
            else if (x.PartName == null) return -1;
            else if (y.PartName == null) return 1;
            else return x.PartName.CompareTo(y.PartName);
        });

        Console.WriteLine("\nAfter sort by name:");
        foreach (Part aPart in parts)
        {
            Console.WriteLine(aPart);
        }
       
        /*
       
            Before sort:
        ID: 1434   Name: regular seat
        ID: 1234   Name: crank arm
        ID: 1634   Name: shift lever
        ID: 1334   Name:
        ID: 1444   Name: banana seat
        ID: 1534   Name: cassette

        After sort by part number:
        ID: 1234   Name: crank arm
        ID: 1334   Name:
        ID: 1434   Name: regular seat
        ID: 1444   Name: banana seat
        ID: 1534   Name: cassette
        ID: 1634   Name: shift lever

        After sort by name:
        ID: 1334   Name:
        ID: 1444   Name: banana seat
        ID: 1534   Name: cassette
        ID: 1234   Name: crank arm
        ID: 1434   Name: regular seat
        ID: 1634   Name: shift lever

         */

    }
}
Imports System.Collections.Generic

' Simple business object. A PartId is used to identify the type of part 
' but the part name can change. 
Public Class Part
    Implements IEquatable(Of Part)
    Implements IComparable(Of Part)
    Public Property PartName() As String
        Get
            Return m_PartName
        End Get
        Set(value As String)
            m_PartName = Value
        End Set
    End Property
    Private m_PartName As String

    Public Property PartId() As Integer
        Get
            Return m_PartId
        End Get
        Set(value As Integer)
            m_PartId = Value
        End Set
    End Property
    Private m_PartId As Integer

    Public Overrides Function ToString() As String
        Return "ID: " & PartId & "   Name: " & PartName
    End Function

    Public Overrides Function Equals(obj As Object) As Boolean
        If obj Is Nothing Then
            Return False
        End If
        Dim objAsPart As Part = TryCast(obj, Part)
        If objAsPart Is Nothing Then
            Return False
        Else
            Return Equals(objAsPart)
        End If
    End Function

    Public Function SortByNameAscending(name1 As String, name2 As String) As Integer

        Return name1.CompareTo(name2)
    End Function

    ' Default comparer for Part.
    Public Function CompareTo(comparePart As Part) As Integer _
            Implements IComparable(Of ListSortVB.Part).CompareTo
        ' A null value means that this object is greater.
        If comparePart Is Nothing Then
            Return 1
        Else

            Return Me.PartId.CompareTo(comparePart.PartId)
        End If
    End Function
    Public Overrides Function GetHashCode() As Integer
        Return PartId
    End Function
    Public Overloads Function Equals(other As Part) As Boolean Implements IEquatable(Of ListSortVB.Part).Equals
        If other Is Nothing Then
            Return False
        End If
        Return (Me.PartId.Equals(other.PartId))
    End Function
    ' Should also override == and != operators.

End Class
Public Class Example
    Public Shared Sub Main()
        ' Create a list of parts.
        Dim parts As New List(Of Part)()

        ' Add parts to the list.
        parts.Add(New Part() With { _
             .PartName = "regular seat", _
             .PartId = 1434 _
        })
        parts.Add(New Part() With { _
             .PartName = "crank arm", _
             .PartId = 1234 _
        })
        parts.Add(New Part() With { _
             .PartName = "shift lever", _
             .PartId = 1634 _
        })


        ' Name intentionally left null.
        parts.Add(New Part() With { _
             .PartId = 1334 _
        })
        parts.Add(New Part() With { _
             .PartName = "banana seat", _
             .PartId = 1444 _
        })
        parts.Add(New Part() With { _
             .PartName = "cassette", _
             .PartId = 1534 _
        })


        ' Write out the parts in the list. This will call the overridden 
        ' ToString method in the Part class.
        Console.WriteLine(vbLf & "Before sort:")
        For Each aPart As Part In parts
            Console.WriteLine(aPart)
        Next


        ' Call Sort on the list. This will use the 
        ' default comparer, which is the Compare method 
        ' implemented on Part.
        parts.Sort()


        Console.WriteLine(vbLf & "After sort by part number:")
        For Each aPart As Part In parts
            Console.WriteLine(aPart)
        Next

        ' This shows calling the Sort(Comparison(T) overload using 
        ' an anonymous delegate method. 
        ' This method treats null as the lesser of two values.
        parts.Sort(Function(x As Part, y As Part)
                             If x.PartName Is Nothing AndAlso y.PartName Is Nothing Then
                                 Return 0
                             ElseIf x.PartName Is Nothing Then
                                 Return -1
                             ElseIf y.PartName Is Nothing Then
                                 Return 1
                             Else
                                 Return x.PartName.CompareTo(y.PartName)
                             End If
                         End Function)
        

        Console.WriteLine(vbLf & "After sort by name:")
        For Each aPart As Part In parts
            Console.WriteLine(aPart)
        Next

        '
        '        
        '            Before sort:
        '            ID: 1434   Name: regular seat
        '            ID: 1234   Name: crank arm
        '            ID: 1634   Name: shift lever
        '            ID: 1334   Name:
        '            ID: 1444   Name: banana seat
        '            ID: 1534   Name: cassette
        '
        '            After sort by part number:
        '            ID: 1234   Name: crank arm
        '            ID: 1334   Name:
        '            ID: 1434   Name: regular seat
        '            ID: 1444   Name: banana seat
        '            ID: 1534   Name: cassette
        '            ID: 1634   Name: shift lever
        '
        '            After sort by name:
        '            ID: 1334   Name:
        '            ID: 1444   Name: banana seat
        '            ID: 1534   Name: cassette
        '            ID: 1234   Name: crank arm
        '            ID: 1434   Name: regular seat
        '            ID: 1634   Name: shift lever

    End Sub
End Class

请参阅See also