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使用LastOrDefault<TSource>(IEnumerable<TSource>)上的空数组。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>

值有时default(TSource)不是你想要使用如果集合不包含任何元素的默认值。Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. 而不是检查不需要的默认值的结果,然后更改有必要,可以使用DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)方法,以指定你想要在集合为空时使用的默认值。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>)若要获取的最后一个元素。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为 1 的数字的月份日期集合是否为空。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. 一个整数,默认值为 0,不对应于该月的任何一天,因为必须指定默认值为 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,检查第一个结果变量为不需要的默认值。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)指定默认值为 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

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

LastOrDefault方法不提供任何方式来指定默认值。The LastOrDefault method does not provide a way to specify a default value. 如果你想要指定默认值以外default(TSource),使用DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)示例部分中所述的方法。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使用LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)通过传入一个谓词。The following code example demonstrates how to use LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) by passing in a predicate. 在第二个方法调用中,在满足以下条件的序列中没有任何元素。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

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

适用于