Enumerable.OrderByDescending Method

Definition

依遞減順序排序序列中的項目。Sorts the elements of a sequence in descending order.

Overloads

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。Sorts the elements of a sequence in descending order by using a specified comparer.

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。Sorts the elements of a sequence in descending order according to a key.

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。Sorts the elements of a sequence in descending order by using a specified comparer.

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);
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)

Type Parameters

TSource

source 項目的類型。The type of the elements of source.

TKey

keySelector 所傳回之索引鍵的型別。The type of the key returned by keySelector.

Parameters

source
IEnumerable<TSource>

要排序的值序列。A sequence of values to order.

keySelector
Func<TSource,TKey>

用來從項目擷取索引鍵的函式。A function to extract a key from an element.

comparer
IComparer<TKey>

用來比較金鑰的 IComparer<T>An IComparer<T> to compare keys.

Returns

IOrderedEnumerable<TSource>

依據索引鍵按遞減順序排序其項目的 IOrderedEnumerable<TElement>An IOrderedEnumerable<TElement> whose elements are sorted in descending order according to a key.

Exceptions

sourcekeySelectornullsource or keySelector is null.

Examples

下列程式碼範例示範如何使用 OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>),以使用轉換函式和自訂比較子,依遞減順序排序序列的元素。The following code example demonstrates how to use OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>) to sort the elements of a sequence in descending order by using a transform function and a custom comparer.

/// <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

Remarks

這個方法是使用延後執行來執行。This method is implemented by using deferred execution. 立即傳回值是一個物件,它會儲存執行動作所需的所有資訊。The immediate return value is an object that stores all the information that is required to perform the action. 在列舉物件之前,不會執行此方法所代表的查詢,方法是直接呼叫其 GetEnumerator 方法,或是在 Visual Basic 中C#使用 Visual 或 For Each 中的 foreachThe query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

若要依專案本身的值排序序列,請在 keySelector中指定識別函式(在C#視覺效果中為x => x,或在 Visual Basic 中為 Function(x) x)。To order a sequence by the values of the elements themselves, specify the identity function (x => x in Visual C# or Function(x) x in Visual Basic) for keySelector.

定義兩種方法來擴充類型 IOrderedEnumerable<TElement>,這是這個方法的傳回型別。Two methods are defined to extend the type IOrderedEnumerable<TElement>, which is the return type of this method. 這兩種方法(亦即 ThenByThenByDescending)可讓您指定其他排序準則來排序序列。These two methods, namely ThenBy and ThenByDescending, enable you to specify additional sort criteria to sort a sequence. ThenByThenByDescending 也會傳回 IOrderedEnumerable<TElement>,這表示可以對 ThenByThenByDescending 進行任意數目的連續呼叫。ThenBy and ThenByDescending also return an IOrderedEnumerable<TElement>, which means any number of consecutive calls to ThenBy or ThenByDescending can be made.

Note

因為 IOrderedEnumerable<TElement> 繼承自 IEnumerable<T>,所以您可以呼叫 OrderByOrderByDescendingThenByThenByDescending呼叫結果的 OrderByOrderByDescendingBecause IOrderedEnumerable<TElement> inherits from IEnumerable<T>, you can call OrderBy or OrderByDescending on the results of a call to OrderBy, OrderByDescending, ThenBy or ThenByDescending. 這麼做引進了新的主要順序,會忽略先前建立的順序。Doing this introduces a new primary ordering that ignores the previously established ordering.

如果 null``comparer,則會使用預設的比較子 Default 來比較索引鍵。If comparer is null, the default comparer Default is used to compare keys.

這個方法會執行穩定的排序;也就是說,如果兩個元素的索引鍵相等,則會保留專案的順序。This method performs a stable sort; that is, if the keys of two elements are equal, the order of the elements is preserved. 相反地,不穩定的排序不會保留具有相同索引鍵的元素順序。In contrast, an unstable sort does not preserve the order of elements that have the same key.

See also

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。Sorts the elements of a sequence in descending order according to a key.

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)

Type Parameters

TSource

source 項目的類型。The type of the elements of source.

TKey

keySelector 所傳回之索引鍵的型別。The type of the key returned by keySelector.

Parameters

source
IEnumerable<TSource>

要排序的值序列。A sequence of values to order.

keySelector
Func<TSource,TKey>

用來從項目擷取索引鍵的函式。A function to extract a key from an element.

Returns

IOrderedEnumerable<TSource>

依據索引鍵按遞減順序排序其項目的 IOrderedEnumerable<TElement>An IOrderedEnumerable<TElement> whose elements are sorted in descending order according to a key.

Exceptions

sourcekeySelectornullsource or keySelector is null.

Remarks

這個方法是使用延後執行來執行。This method is implemented by using deferred execution. 立即傳回值是一個物件,它會儲存執行動作所需的所有資訊。The immediate return value is an object that stores all the information that is required to perform the action. 在列舉物件之前,不會執行此方法所代表的查詢,方法是直接呼叫其 GetEnumerator 方法,或是在 Visual Basic 中C#使用 Visual 或 For Each 中的 foreachThe query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

若要依專案本身的值排序序列,請在 keySelector中指定識別函式(在C#視覺效果中為x => x,或在 Visual Basic 中為 Function(x) x)。To order a sequence by the values of the elements themselves, specify the identity function (x => x in Visual C# or Function(x) x in Visual Basic) for keySelector.

如需這個方法的範例,請參閱 OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>)For an example of this method, see OrderByDescending<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>).

定義兩種方法來擴充類型 IOrderedEnumerable<TElement>,這是這個方法的傳回型別。Two methods are defined to extend the type IOrderedEnumerable<TElement>, which is the return type of this method. 這兩種方法(亦即 ThenByThenByDescending)可讓您指定其他排序準則來排序序列。These two methods, namely ThenBy and ThenByDescending, enable you to specify additional sort criteria to sort a sequence. ThenByThenByDescending 也會傳回 IOrderedEnumerable<TElement>,這表示可以對 ThenByThenByDescending 進行任意數目的連續呼叫。ThenBy and ThenByDescending also return an IOrderedEnumerable<TElement>, which means any number of consecutive calls to ThenBy or ThenByDescending can be made.

Note

因為 IOrderedEnumerable<TElement> 繼承自 IEnumerable<T>,所以您可以呼叫 OrderByOrderByDescendingThenByThenByDescending呼叫結果的 OrderByOrderByDescendingBecause IOrderedEnumerable<TElement> inherits from IEnumerable<T>, you can call OrderBy or OrderByDescending on the results of a call to OrderBy, OrderByDescending, ThenBy or ThenByDescending. 這麼做引進了新的主要順序,會忽略先前建立的順序。Doing this introduces a new primary ordering that ignores the previously established ordering.

這個方法會使用預設的比較子 Default來比較索引鍵。This method compares keys by using the default comparer Default.

這個方法會執行穩定的排序;也就是說,如果兩個元素的索引鍵相等,則會保留專案的順序。This method performs a stable sort; that is, if the keys of two elements are equal, the order of the elements is preserved. 相反地,不穩定的排序不會保留具有相同索引鍵的元素順序。In contrast, an unstable sort does not preserve the order of elements that have the same key.

在查詢運算式語法中,orderby descending (Visual C#)或 Order By Descending (Visual Basic)子句會轉譯成 OrderByDescending的調用。In query expression syntax, an orderby descending (Visual C#) or Order By Descending (Visual Basic) clause translates to an invocation of OrderByDescending.

See also

Applies to