Enumerable.LastOrDefault Method

Definition

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.

Overloads

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>)

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>)

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

Type Parameters

TSource

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

Parameters

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.

Returns

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>.

Exceptions

sourcenullsource is null.

Examples

下面的代码示例演示如何对空数组使用 LastOrDefault<TSource>(IEnumerable<TSource>)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.
Console.WriteLine(IIf(String.IsNullOrEmpty(last),
       "<string is Nothing or empty>",
       last))

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

有时,如果集合不包含任何元素,则 default(TSource) 的值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,然后根据需要进行更改。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>) 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. 第二个结果变量是通过使用 DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) 指定默认值1获取的。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
Console.WriteLine($"The value of the lastDay1 variable is {lastDay1}")

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim lastDay2 As Integer = daysOfMonth.DefaultIfEmpty(1).Last()
Console.WriteLine($"The value of the lastDay2 variable is {lastDay2}")

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

Remarks

引用和可以为 null 的类型的默认值为 nullThe 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 default(TSource)以外的默认值,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>)

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

Type Parameters

TSource

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

Parameters

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.

Returns

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.

Exceptions

sourcepredicatenullsource or predicate is null.

Examples

下面的代码示例演示如何通过传入谓词来使用 LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)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.
Console.WriteLine(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]

Remarks

引用和可以为 null 的类型的默认值为 nullThe default value for reference and nullable types is null.

Applies to