Enumerable.OrderByDescending 方法

定义

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。

返回

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

例外

sourcekeySelectornull

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其方法或通过在 C# For Eachforeach 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 keySelector) 中指定标识函数 (x => x

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。 通过这两种方法(即 ThenByThenByDescending),可以指定其他排序条件来对序列进行排序。 ThenByThenByDescending 还返回 , IOrderedEnumerable<TElement>这意味着可以进行任意数量的连续调用 ThenByThenByDescending

注意

由于 IOrderedEnumerable<TElement> 继承自 IEnumerable<T>,因此可以在对 OrderByOrderByDescendingThenByOrderByDescending 的调用结果上调用 OrderByThenByDescending。 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比较键。

此方法执行稳定排序;也就是说,如果两个元素的键相等,则保留元素的顺序。 相反,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>

返回

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

例外

sourcekeySelectornull

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

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其方法或通过在 C# For Eachforeach 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 keySelector) 中指定标识函数 (x => x

定义了两种方法来扩展类型 IOrderedEnumerable<TElement>,这是此方法的返回类型。 通过这两种方法(即 ThenByThenByDescending),可以指定其他排序条件来对序列进行排序。 ThenByThenByDescending 还返回 , IOrderedEnumerable<TElement>这意味着可以进行任意数量的连续调用 ThenByThenByDescending

注意

由于 IOrderedEnumerable<TElement> 继承自 IEnumerable<T>,因此可以在对 OrderByOrderByDescendingThenByOrderByDescending 的调用结果上调用 OrderByThenByDescending。 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。

如果 comparernull,则使用默认比较器 Default 来比较键。

此方法执行稳定排序;也就是说,如果两个元素的键相等,则保留元素的顺序。 相反,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