Enumerable.OrderByDescending 方法

参考

定义

命名空间:: System.Linq

程序集:: System.Linq.dll

程序集:: System.Core.dll

程序集:: netstandard.dll

重要

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按降序对序列的元素排序。

重载

OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>)	根据键按降序对序列的元素进行排序。
OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>)	使用指定的比较器按降序对序列的元素排序。

OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>)

Source:: OrderBy.cs

Source:: OrderBy.cs

Source:: OrderBy.cs

根据键按降序对序列的元素进行排序。

public:
generic <typename TSource, typename TKey>
[System::Runtime::CompilerServices::Extension]
 static System::Linq::IOrderedEnumerable<TSource> ^ OrderByDescending(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, TKey> ^ keySelector);

public static System.Linq.IOrderedEnumerable<TSource> OrderByDescending<TSource,TKey> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector);

static member OrderByDescending : seq<'Source> * Func<'Source, 'Key> -> System.Linq.IOrderedEnumerable<'Source>

<Extension()>
Public Function OrderByDescending(Of TSource, TKey) (source As IEnumerable(Of TSource), keySelector As Func(Of TSource, TKey)) As IOrderedEnumerable(Of TSource)

类型参数

TSource

source 的元素类型。

TKey

keySelector 返回的键的类型。

参数

source: IEnumerable<TSource>

一个要排序的值序列。

keySelector: Func<TSource,TKey>

用于从元素中提取键的函数。

IOrderedEnumerable<TSource>

一个 IOrderedEnumerable<TElement>，将根据键按降序对其元素进行排序。

例外

ArgumentNullException

source 或 keySelector 为 null。

注解

此方法通过使用延迟执行来实现。即时返回值是一个对象，用于存储执行操作所需的所有信息。在通过直接调用GetEnumerator其方法或在 C# For Each 或 foreach Visual Basic 中使用来枚举对象之前，不会执行此方法表示的查询。

若要按元素本身的值对序列进行排序，请在 C# 或 Function(x) x Visual Basic) 中指定标识函数 (。x => xkeySelector

有关此方法的示例，请参阅 OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>)。

定义了两种方法来扩展类型 IOrderedEnumerable<TElement>，这是此方法的返回类型。这两种方法（即 ThenBy 和 ThenByDescending）使你可以指定其他排序条件来对序列进行排序。 ThenBy和 ThenByDescending 还返回，IOrderedEnumerable<TElement>这意味着可以对或 ThenByDescending 进行任意次数的连续调用ThenBy。

注意

由于 IOrderedEnumerable<TElement> 继承自 IEnumerable<T>，因此可以在对、 OrderByDescending ThenBy 或 OrderByDescending 的调用结果中调用 OrderBy OrderBy或 ThenByDescending。这样做会引入一个新的主排序，该排序会忽略以前建立的排序。

此方法使用默认比较器 Default来比较键。

此方法执行稳定的排序;也就是说，如果两个元素的键相等，则保留元素的顺序。相比之下，不稳定排序不会保留具有相同键的元素的顺序。

在查询表达式语法中， orderby descending (C#) 或 Order By Descending (Visual Basic) 子句将转换为的调用 OrderByDescending。

另请参阅

适用于

OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>)

Source:: OrderBy.cs

Source:: OrderBy.cs

Source:: OrderBy.cs

使用指定的比较器按降序对序列的元素排序。

public:
generic <typename TSource, typename TKey>
[System::Runtime::CompilerServices::Extension]
 static System::Linq::IOrderedEnumerable<TSource> ^ OrderByDescending(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, TKey> ^ keySelector, System::Collections::Generic::IComparer<TKey> ^ comparer);

public static System.Linq.IOrderedEnumerable<TSource> OrderByDescending<TSource,TKey> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, System.Collections.Generic.IComparer<TKey> comparer);

public static System.Linq.IOrderedEnumerable<TSource> OrderByDescending<TSource,TKey> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, System.Collections.Generic.IComparer<TKey>? comparer);

static member OrderByDescending : seq<'Source> * Func<'Source, 'Key> * System.Collections.Generic.IComparer<'Key> -> System.Linq.IOrderedEnumerable<'Source>

<Extension()>
Public Function OrderByDescending(Of TSource, TKey) (source As IEnumerable(Of TSource), keySelector As Func(Of TSource, TKey), comparer As IComparer(Of TKey)) As IOrderedEnumerable(Of TSource)

类型参数

TSource

source 的元素类型。

TKey

keySelector 返回的键的类型。

参数

source: IEnumerable<TSource>

一个要排序的值序列。

keySelector: Func<TSource,TKey>

用于从元素中提取键的函数。

comparer: IComparer<TKey>

用于比较键的 IComparer<T>。

IOrderedEnumerable<TSource>

一个 IOrderedEnumerable<TElement>，将根据键按降序对其元素进行排序。

例外

ArgumentNullException

source 或 keySelector 为 null。

示例

下面的代码示例演示如何使用 OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>) 转换函数和自定义比较器按降序对序列的元素进行排序。

/// <summary>
/// This IComparer class sorts by the fractional part of the decimal number.
/// </summary>
public class SpecialComparer : IComparer<decimal>
{
    /// <summary>
    /// Compare two decimal numbers by their fractional parts.
    /// </summary>
    /// <param name="d1">The first decimal to compare.</param>
    /// <param name="d2">The second decimal to compare.</param>
    /// <returns>1 if the first decimal's fractional part
    /// is greater than the second decimal's fractional part,
    /// -1 if the first decimal's fractional
    /// part is less than the second decimal's fractional part,
    /// or the result of calling Decimal.Compare()
    /// if the fractional parts are equal.</returns>
    public int Compare(decimal d1, decimal d2)
    {
        decimal fractional1, fractional2;

        // Get the fractional part of the first number.
        try
        {
            fractional1 = decimal.Remainder(d1, decimal.Floor(d1));
        }
        catch (DivideByZeroException)
        {
            fractional1 = d1;
        }
        // Get the fractional part of the second number.
        try
        {
            fractional2 = decimal.Remainder(d2, decimal.Floor(d2));
        }
        catch (DivideByZeroException)
        {
            fractional2 = d2;
        }

        if (fractional1 == fractional2)
            return Decimal.Compare(d1, d2);
        else if (fractional1 > fractional2)
            return 1;
        else
            return -1;
    }
}

public static void OrderByDescendingEx1()
{
    List<decimal> decimals =
        new List<decimal> { 6.2m, 8.3m, 0.5m, 1.3m, 6.3m, 9.7m };

    IEnumerable<decimal> query =
        decimals.OrderByDescending(num =>
                                       num, new SpecialComparer());

    foreach (decimal num in query)
    {
        Console.WriteLine(num);
    }
}

/*
 This code produces the following output:

 9.7
 0.5
 8.3
 6.3
 1.3
 6.2
*/

' This class provides a custom implementation
' of the IComparer.Compare() method.
Class SpecialComparer
    Implements IComparer(Of Decimal)
    ''' <summary>
    ''' Compare two decimal numbers by their fractional parts.
    ''' </summary>
    ''' <param name="d1">The first decimal to compare.</param>
    ''' <param name="d2">The second decimal to compare.</param>
    ''' <returns>1 if the first decimal's fractional part is greater than
    ''' the second decimal's fractional part,
    ''' -1 if the first decimal's fractional
    ''' part is less than the second decimal's fractional part,
    ''' or the result of calling Decimal.Compare()
    ''' if the fractional parts are equal.</returns>
    Function Compare(ByVal d1 As Decimal, ByVal d2 As Decimal) As Integer _
    Implements IComparer(Of Decimal).Compare

        Dim fractional1 As Decimal
        Dim fractional2 As Decimal

        ' Get the fractional part of the first number.
        Try
            fractional1 = Decimal.Remainder(d1, Decimal.Floor(d1))
        Catch ex As DivideByZeroException
            fractional1 = d1
        End Try

        ' Get the fractional part of the second number.
        Try
            fractional2 = Decimal.Remainder(d2, Decimal.Floor(d2))
        Catch ex As DivideByZeroException
            fractional2 = d2
        End Try

        If (fractional1 = fractional2) Then
            ' The fractional parts are equal, so compare the entire numbers.
            Return Decimal.Compare(d1, d2)
        ElseIf (fractional1 > fractional2) Then
            Return 1
        Else
            Return -1
        End If
    End Function
End Class

Sub OrderByDescendingEx1()
    ' Create a list of decimal values.
    Dim decimals As New List(Of Decimal)(New Decimal() _
                                     {6.2D, 8.3D, 0.5D, 1.3D, 6.3D, 9.7D})

    ' Order the elements of the list by passing
    ' in the custom IComparer class.
    Dim query As IEnumerable(Of Decimal) =
    decimals.OrderByDescending(Function(num) num,
                               New SpecialComparer())

    Dim output As New System.Text.StringBuilder
    For Each num As Decimal In query
        output.AppendLine(num)
    Next

    ' Display the output.
    Console.WriteLine(output.ToString())
End Sub

' This code produces the following output:
'
' 9.7
' 0.5
' 8.3
' 6.3
' 1.3
' 6.2

注解

若要按元素本身的值对序列进行排序，请在 C# 或 Function(x) x Visual Basic) 中指定标识函数 (。x => xkeySelector

注意

如果 comparer 为 null，则使用默认比较器 Default 来比较键。

此方法执行稳定的排序;也就是说，如果两个元素的键相等，则保留元素的顺序。相比之下，不稳定排序不会保留具有相同键的元素的顺序。

另请参阅

适用于

Enumerable.OrderByDescending 方法

定义

重载

OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>)

类型参数

参数

返回

例外

注解

另请参阅

适用于

OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>)

类型参数

参数

返回

例外

示例

注解

另请参阅

适用于

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