Queryable.OrderByDescending 方法

定義

依遞減順序排序序列中的項目。

多載

OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<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>(IQueryable<TSource>, Expression<Func<TSource,TKey>>)

依據索引鍵,按遞減順序排序序列中的項目。

OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<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。

public:
generic <typename TSource, typename TKey>
[System::Runtime::CompilerServices::Extension]
 static System::Linq::IOrderedQueryable<TSource> ^ OrderByDescending(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, TKey> ^> ^ keySelector, System::Collections::Generic::IComparer<TKey> ^ comparer);
public static System.Linq.IOrderedQueryable<TSource> OrderByDescending<TSource,TKey> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,TKey>> keySelector, System.Collections.Generic.IComparer<TKey> comparer);
public static System.Linq.IOrderedQueryable<TSource> OrderByDescending<TSource,TKey> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,TKey>> keySelector, System.Collections.Generic.IComparer<TKey>? comparer);
static member OrderByDescending : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, 'Key>> * System.Collections.Generic.IComparer<'Key> -> System.Linq.IOrderedQueryable<'Source>
<Extension()>
Public Function OrderByDescending(Of TSource, TKey) (source As IQueryable(Of TSource), keySelector As Expression(Of Func(Of TSource, TKey)), comparer As IComparer(Of TKey)) As IOrderedQueryable(Of TSource)

類型參數

TSource

source 項目的類型。

TKey

keySelector 表示之函式所傳回索引鍵的型別。

參數

source
IQueryable<TSource>

要排序的值序列。

keySelector
Expression<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

IOrderedQueryable<TSource>

依據索引鍵按遞減順序排序其項目的 IOrderedQueryable<T>

例外狀況

sourcekeySelectorcomparernull

範例

下列程式碼範例示範如何使用 OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<Func<TSource,TKey>>, IComparer<TKey>) 自訂比較子,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 };

    // Sort the decimal values in descending order
    // by using a custom comparer.
    IEnumerable<decimal> query =
        decimals.AsQueryable()
        .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
*/
''' <summary>
''' This class provides a custom implementation of the
''' IComparer.Compare() method. It sorts by the fractional part of the decimal number.
''' </summary>
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.AsQueryable(). _
        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.
    MsgBox(output.ToString())
End Sub

' This code produces the following output:

' 9.7
' 0.5
' 8.3
' 6.3
' 1.3
' 6.2

備註

這個方法至少有一個類型 Expression<TDelegate> 參數,其類型引數為其中一個 Func<T,TResult> 型別。 針對這些參數,您可以傳入 Lambda 運算式,並將它編譯為 Expression<TDelegate>

方法 OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<Func<TSource,TKey>>, IComparer<TKey>) 會產生 , MethodCallExpression 表示 OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<Func<TSource,TKey>>, IComparer<TKey>) 呼叫本身為建構的泛型方法。 然後,它會將 傳遞 MethodCallExpressionCreateQuery<TElement>(Expression) 參數的 屬性所 Provider 表示的 source 方法 IQueryProvider 。 呼叫 CreateQuery<TElement>(Expression) 的結果會轉換成類型 IOrderedQueryable<T> 並傳回。

執行表示呼叫 OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<Func<TSource,TKey>>, IComparer<TKey>) 的運算式樹狀結構所產生的查詢行為,取決於參數類型的 source 實作。 預期的行為是它會根據 叫用 的每個元素取得 keySelector 的索引鍵,依遞減順序排序 sourcesource 元素。 參數 comparer 是用來比較索引鍵。

適用於

OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<Func<TSource,TKey>>)

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::IOrderedQueryable<TSource> ^ OrderByDescending(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, TKey> ^> ^ keySelector);
public static System.Linq.IOrderedQueryable<TSource> OrderByDescending<TSource,TKey> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,TKey>> keySelector);
static member OrderByDescending : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, 'Key>> -> System.Linq.IOrderedQueryable<'Source>
<Extension()>
Public Function OrderByDescending(Of TSource, TKey) (source As IQueryable(Of TSource), keySelector As Expression(Of Func(Of TSource, TKey))) As IOrderedQueryable(Of TSource)

類型參數

TSource

source 項目的類型。

TKey

keySelector 表示之函式所傳回索引鍵的型別。

參數

source
IQueryable<TSource>

要排序的值序列。

keySelector
Expression<Func<TSource,TKey>>

用來從項目擷取索引鍵的函式。

傳回

IOrderedQueryable<TSource>

依據索引鍵按遞減順序排序其項目的 IOrderedQueryable<T>

例外狀況

sourcekeySelectornull

備註

這個方法至少有一個類型 Expression<TDelegate> 參數,其類型引數為其中一個 Func<T,TResult> 型別。 針對這些參數,您可以傳入 Lambda 運算式,並將它編譯為 Expression<TDelegate>

方法 OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<Func<TSource,TKey>>) 會產生 , MethodCallExpression 表示 OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<Func<TSource,TKey>>) 呼叫本身為建構的泛型方法。 然後,它會將 傳遞 MethodCallExpressionCreateQuery<TElement>(Expression) 參數的 屬性所 Provider 表示的 source 方法 IQueryProvider 。 呼叫 CreateQuery<TElement>(Expression) 的結果會轉換成類型 IOrderedQueryable<T> 並傳回。

執行表示呼叫 OrderByDescending<TSource,TKey>(IQueryable<TSource>, Expression<Func<TSource,TKey>>) 的運算式樹狀結構所產生的查詢行為,取決於參數類型的 source 實作。 預期的行為是它會根據 叫用 的每個元素取得 keySelector 的索引鍵,依遞減順序排序 sourcesource 元素。

適用於