Enumerable.OrderByDescending 方法

參考

定義

命名空間:: System.Linq

組件:: System.Linq.dll

組件:: System.Core.dll

組件:: netstandard.dll

重要

部分資訊涉及發行前產品，在發行之前可能會有大幅修改。 Microsoft 對此處提供的資訊，不做任何明確或隱含的瑕疵擔保。

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

IOrderedEnumerable<TSource>

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

例外狀況

ArgumentNullException

source 或 keySelector 為 null。

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 ThenBy 和 ThenByDescending，可讓您指定其他排序準則來排序序列。 ThenBy和 ThenByDescending 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、 OrderByDescending ThenBy 或的結果上呼叫 OrderByDescending OrderBy OrderBy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

IOrderedEnumerable<TSource>

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

例外狀況

ArgumentNullException

source 或 keySelector 為 null。

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

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。

注意

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

另請參閱

適用於

Enumerable.OrderByDescending 方法

定義

多載

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

類型參數

參數

傳回

例外狀況

備註

另請參閱

適用於

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

類型參數

參數

傳回

例外狀況

範例

備註

另請參閱

適用於

意見反應

其他資源