Enumerable.LastOrDefault Enumerable.LastOrDefault Enumerable.LastOrDefault Enumerable.LastOrDefault Method

定义

返回序列中的最后一个元素;如果未找到该元素,则返回默认值。Returns the last element of a sequence, or a default value if no element is found.

重载

LastOrDefault<TSource>(IEnumerable<TSource>) LastOrDefault<TSource>(IEnumerable<TSource>) LastOrDefault<TSource>(IEnumerable<TSource>) LastOrDefault<TSource>(IEnumerable<TSource>)

返回序列中的最后一个元素;如果序列中不包含任何元素,则返回默认值。Returns the last element of a sequence, or a default value if the sequence contains no elements.

LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)

返回序列中满足条件的最后一个元素;如果未找到这样的元素,则返回默认值。Returns the last element of a sequence that satisfies a condition or a default value if no such element is found.

LastOrDefault<TSource>(IEnumerable<TSource>) LastOrDefault<TSource>(IEnumerable<TSource>) LastOrDefault<TSource>(IEnumerable<TSource>) LastOrDefault<TSource>(IEnumerable<TSource>)

返回序列中的最后一个元素;如果序列中不包含任何元素,则返回默认值。Returns the last element of a sequence, or a default value if the sequence contains no elements.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource LastOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source);
public static TSource LastOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);
static member LastOrDefault : seq<'Source> -> 'Source
<Extension()>
Public Function LastOrDefault(Of TSource) (source As IEnumerable(Of TSource)) As TSource

类型参数

TSource

source 的元素类型。The type of the elements of source.

参数

source
IEnumerable<TSource>

要返回其最后一个元素的 IEnumerable<T>An IEnumerable<T> to return the last element of.

返回

TSource TSource TSource TSource

如果源序列为空,则为 default(TSource);否则为 IEnumerable<T> 中的最后一个元素。default(TSource) if the source sequence is empty; otherwise, the last element in the IEnumerable<T>.

异常

示例

下面的代码示例演示如何对空LastOrDefault<TSource>(IEnumerable<TSource>)数组使用。The following code example demonstrates how to use LastOrDefault<TSource>(IEnumerable<TSource>) on an empty array.

string[] fruits = { };
string last = fruits.LastOrDefault();
Console.WriteLine(
    String.IsNullOrEmpty(last) ? "<string is null or empty>" : last);

/*
 This code produces the following output:

 <string is null or empty>
*/
' Create an empty array.
Dim fruits() As String = {}

' Get the last item in the array, or a
' default value if there are no items.
Dim last As String = fruits.LastOrDefault()

' Display the result.
MsgBox(IIf(String.IsNullOrEmpty(last),
       "<string is Nothing or empty>",
       last))

' This code produces the following output:
'
' <string is Nothing or empty>

有时, 如果集合default(TSource)不包含任何元素, 则的值将不是要使用的默认值。Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. 您可以使用DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)方法来指定要在集合为空时使用的默认值, 而不是检查不需要的默认值的结果, 并在必要时对其进行更改。Instead of checking the result for the unwanted default value and then changing it if necessary, you can use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. 然后, 调用Last<TSource>(IEnumerable<TSource>)以获取最后一个元素。Then, call Last<TSource>(IEnumerable<TSource>) to obtain the last element. 下面的代码示例使用这两种方法来获取默认值 1 (如果该月中的数字天数集合为空)。The following code example uses both techniques to obtain a default value of 1 if a collection of numeric days of the month is empty. 由于整数的默认值为 0 (不对应于月中的任何一天), 因此必须将默认值指定为1。Because the default value for an integer is 0, which does not correspond to any day of the month, the default value must be specified as 1 instead. 查询执行完毕后, 将检查第一个结果变量是否存在不需要的默认值。The first result variable is checked for the unwanted default value after the query has finished executing. 第二个结果变量通过使用DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)来指定默认值1。The second result variable is obtained by using DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) to specify a default value of 1.

List<int> daysOfMonth = new List<int> { };

// Setting the default value to 1 after the query.
int lastDay1 = daysOfMonth.LastOrDefault();
if (lastDay1 == 0)
{
    lastDay1 = 1;
}
Console.WriteLine("The value of the lastDay1 variable is {0}", lastDay1);

// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int lastDay2 = daysOfMonth.DefaultIfEmpty(1).Last();
Console.WriteLine("The value of the lastDay2 variable is {0}", lastDay2);

/*
 This code produces the following output:
 
 The value of the lastDay1 variable is 1
 The value of the lastDay2 variable is 1
*/
Dim daysOfMonth As New List(Of Integer)(New Integer() {})

' Setting the default value to 1 after the query.
Dim lastDay1 As Integer = daysOfMonth.LastOrDefault()
If lastDay1 = 0 Then
    lastDay1 = 1
End If
MsgBox(String.Format("The value of the lastDay1 variable is {0}", lastDay1))

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim lastDay2 As Integer = daysOfMonth.DefaultIfEmpty(1).Last()
MsgBox(String.Format("The value of the lastDay2 variable is {0}", lastDay2))

' This code produces the following output:
'
' The value of the lastDay1 variable is 1
' The value of the lastDay2 variable is 1

注解

引用类型和可以为 null 的类型的null默认值为。The default value for reference and nullable types is null.

LastOrDefault方法不提供指定默认值的方法。The LastOrDefault method does not provide a way to specify a default value. 如果要指定之外的默认值default(TSource), 请使用 "示例" 部分中所述的DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)方法。If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method as described in the Example section.

LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)

返回序列中满足条件的最后一个元素;如果未找到这样的元素,则返回默认值。Returns the last element of a sequence that satisfies a condition or a default value if no such element is found.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource LastOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, bool> ^ predicate);
public static TSource LastOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate);
static member LastOrDefault : seq<'Source> * Func<'Source, bool> -> 'Source
<Extension()>
Public Function LastOrDefault(Of TSource) (source As IEnumerable(Of TSource), predicate As Func(Of TSource, Boolean)) As TSource

类型参数

TSource

source 的元素类型。The type of the elements of source.

参数

source
IEnumerable<TSource>

要从中返回元素的 IEnumerable<T>An IEnumerable<T> to return an element from.

predicate
Func<TSource,Boolean>

用于测试每个元素是否满足条件的函数。A function to test each element for a condition.

返回

TSource TSource TSource TSource

如果序列为空或没有元素通过谓词函数中的测试,则为 default(TSource);否则,为通过谓词函数中的测试的最后一个元素。default(TSource) if the sequence is empty or if no elements pass the test in the predicate function; otherwise, the last element that passes the test in the predicate function.

异常

sourcepredicatenullsource or predicate is null.

示例

下面的代码示例演示如何通过传入LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)谓词来使用。The following code example demonstrates how to use LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) by passing in a predicate. 在第二次调用方法时, 序列中没有满足条件的元素。In the second call to the method, there is no element in the sequence that satisfies the condition.

double[] numbers = { 49.6, 52.3, 51.0, 49.4, 50.2, 48.3 };

double last50 = numbers.LastOrDefault(n => Math.Round(n) == 50.0);

Console.WriteLine("The last number that rounds to 50 is {0}.", last50);

double last40 = numbers.LastOrDefault(n => Math.Round(n) == 40.0);

Console.WriteLine(
    "The last number that rounds to 40 is {0}.",
    last40 == 0.0 ? "<DOES NOT EXIST>" : last40.ToString());

/*
 This code produces the following output:

 The last number that rounds to 50 is 50.2.
 The last number that rounds to 40 is <DOES NOT EXIST>.
*/
' Create an array of doubles.
Dim numbers() As Double = {49.6, 52.3, 51.0, 49.4, 50.2, 48.3}

' Get the last item whose value rounds to 50.0.
Dim number50 As Double =
numbers.LastOrDefault(Function(n) Math.Round(n) = 50.0)

Dim output As New System.Text.StringBuilder
output.AppendLine("The last number that rounds to 50 is " & number50)

' Get the last item whose value rounds to 40.0.
Dim number40 As Double =
numbers.LastOrDefault(Function(n) Math.Round(n) = 40.0)

Dim text As String = IIf(number40 = 0.0,
                     "[DOES NOT EXIST]",
                     number40.ToString())
output.AppendLine("The last number that rounds to 40 is " & text)

' Display the output.
MsgBox(output.ToString)

' This code produces the following output:
'
' The last number that rounds to 50 is 50.2
' The last number that rounds to 40 is [DOES NOT EXIST]

注解

引用类型和可以为 null 的类型的null默认值为。The default value for reference and nullable types is null.

适用于