共用方式為


Enumerable.OrderByDescending 方法

定義

依遞減順序排序序列中的項目。

多載

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

依據索引鍵,按遞減順序排序序列中的項目。

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

使用指定的比較子,依遞減順序排序序列中的項目。

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

來源:
OrderBy.cs
來源:
OrderBy.cs
來源:
OrderBy.cs

依據索引鍵,按遞減順序排序序列中的項目。

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)

類型參數

TSource

source 項目的類型。

TKey

keySelector 所傳回之索引鍵的型別。

參數

source
IEnumerable<TSource>

要排序的值序列。

keySelector
Func<TSource,TKey>

用來從項目擷取索引鍵的函式。

傳回

依據索引鍵按遞減順序排序其項目的 IOrderedEnumerable<TElement>

例外狀況

sourcekeySelectornull

備註

此方法是使用延後執行來實作。 立即傳回值是物件,可儲存執行動作所需的所有資訊。 除非直接在 GetEnumerator C# 或 Visual Basic 中使用 foreach 來列舉對象,否則 For Each 不會執行這個方法所表示的查詢。

若要依元素本身的值排序序列,請在 C# 或 Function(x) x Visual Basic) 中指定識別函x => x式 (keySelector

如需這個方法的範例,請參閱 OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>)

定義兩種方法來擴充 型 IOrderedEnumerable<TElement>別 ,這是這個方法的傳回型別。 這兩種方法,也就是 ThenByThenByDescending,可讓您指定其他排序準則來排序序列。 ThenByThenByDescending 也會傳回 IOrderedEnumerable<TElement>,這表示可以對 或 ThenByDescending 進行任意數目的連續呼叫ThenBy

注意

因為 IOrderedEnumerable<TElement> 繼承自 IEnumerable<T>,所以您可以在呼叫 、 OrderByDescendingThenBy 或 的結果上呼叫 OrderByDescendingOrderByOrderBy或 。ThenByDescending 這麼做會引進新的主要順序,以忽略先前建立的排序。

此方法會使用預設比較子 Default來比較索引鍵。

這個方法會執行穩定的排序;也就是說,如果兩個專案的索引鍵相等,則會保留元素的順序。 相反地,不穩定的排序不會保留具有相同索引鍵的項目順序。

在查詢表達式語法中, orderby descending (C#) 或 Order By Descending (Visual Basic) 子句會轉譯為 的 OrderByDescending調用。

另請參閱

適用於

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

來源:
OrderBy.cs
來源:
OrderBy.cs
來源:
OrderBy.cs

使用指定的比較子,依遞減順序排序序列中的項目。

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)

類型參數

TSource

source 項目的類型。

TKey

keySelector 所傳回之索引鍵的型別。

參數

source
IEnumerable<TSource>

要排序的值序列。

keySelector
Func<TSource,TKey>

用來從項目擷取索引鍵的函式。

comparer
IComparer<TKey>

用來比較金鑰的 IComparer<T>

傳回

依據索引鍵按遞減順序排序其項目的 IOrderedEnumerable<TElement>

例外狀況

sourcekeySelectornull

範例

下列程式代碼範例示範如何使用 OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>) 轉換函式和自定義比較子,以遞減順序排序序列的專案。

/// <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

備註

此方法是使用延後執行來實作。 立即傳回值是物件,可儲存執行動作所需的所有資訊。 除非直接在 GetEnumerator C# 或 Visual Basic 中使用 foreach 來列舉對象,否則 For Each 不會執行這個方法所表示的查詢。

若要依元素本身的值排序序列,請在 C# 或 Function(x) x Visual Basic) 中指定識別函x => x式 (keySelector

定義兩種方法來擴充 型 IOrderedEnumerable<TElement>別 ,這是這個方法的傳回型別。 這兩種方法,也就是 ThenByThenByDescending,可讓您指定其他排序準則來排序序列。 ThenByThenByDescending 也會傳回 IOrderedEnumerable<TElement>,這表示可以對 或 ThenByDescending 進行任意數目的連續呼叫ThenBy

注意

因為 IOrderedEnumerable<TElement> 繼承自 IEnumerable<T>,所以您可以在呼叫 、 OrderByDescendingThenBy 或 的結果上呼叫 OrderByDescendingOrderByOrderBy或 。ThenByDescending 這麼做會引進新的主要順序,以忽略先前建立的排序。

如果 comparernull,則會使用預設比較子 Default 來比較索引鍵。

這個方法會執行穩定的排序;也就是說,如果兩個專案的索引鍵相等,則會保留元素的順序。 相反地,不穩定的排序不會保留具有相同索引鍵的項目順序。

另請參閱

適用於