Enumerable.First Enumerable.First Enumerable.First Enumerable.First Method

定义

返回序列中的第一个元素。Returns the first element of a sequence.

重载

First<TSource>(IEnumerable<TSource>) First<TSource>(IEnumerable<TSource>) First<TSource>(IEnumerable<TSource>) First<TSource>(IEnumerable<TSource>)

返回序列中的第一个元素。Returns the first element of a sequence.

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

返回序列中满足指定条件的第一个元素。Returns the first element in a sequence that satisfies a specified condition.

First<TSource>(IEnumerable<TSource>) First<TSource>(IEnumerable<TSource>) First<TSource>(IEnumerable<TSource>) First<TSource>(IEnumerable<TSource>)

返回序列中的第一个元素。Returns the first element of a sequence.

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

要返回其第一个元素的 IEnumerable<T>The IEnumerable<T> to return the first element of.

返回

TSource TSource TSource TSource

返回指定序列中的第一个元素。The first element in the specified sequence.

异常

示例

下面的代码示例演示如何使用First<TSource>(IEnumerable<TSource>)返回数组的第一个元素。The following code example demonstrates how to use First<TSource>(IEnumerable<TSource>) to return the first element of an array.

int[] numbers = { 9, 34, 65, 92, 87, 435, 3, 54, 
                    83, 23, 87, 435, 67, 12, 19 };

int first = numbers.First();

Console.WriteLine(first);

/*
 This code produces the following output:

 9
*/
' Create an array of integers.
Dim numbers() As Integer =
{9, 34, 65, 92, 87, 435, 3, 54, 83, 23, 87, 435, 67, 12, 19}

' Select the first element in the array.
Dim first As Integer = numbers.First()

' Display the output.
MsgBox(first)

' This code produces the following output:
'
' 9

注解

First<TSource>(IEnumerable<TSource>) 如果source不包含任何元素, 则该方法将引发异常。The First<TSource>(IEnumerable<TSource>) method throws an exception if source contains no elements. 若要改为在源序列为空时返回默认值, 请FirstOrDefault使用方法。To instead return a default value when the source sequence is empty, use the FirstOrDefault method.

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

返回序列中满足指定条件的第一个元素。Returns the first element in a sequence that satisfies a specified condition.

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

序列中通过指定谓词函数中的测试的第一个元素。The first element in the sequence that passes the test in the specified predicate function.

异常

sourcepredicatenullsource or predicate is null.

元素均不满足 predicate 中的条件。No element satisfies the condition in predicate.

-or- 源序列为空。The source sequence is empty.

示例

下面的代码示例演示如何使用First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)返回满足某个条件的数组的第一个元素。The following code example demonstrates how to use First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) to return the first element of an array that satisfies a condition.

int[] numbers = { 9, 34, 65, 92, 87, 435, 3, 54, 
                    83, 23, 87, 435, 67, 12, 19 };

int first = numbers.First(number => number > 80);

Console.WriteLine(first);

/*
 This code produces the following output:

 92
*/
' Create an array of integers.
Dim numbers() As Integer =
{9, 34, 65, 92, 87, 435, 3, 54, 83, 23, 87, 435, 67, 12, 19}

' Select the first element in the array whose value is greater than 80.
Dim first As Integer = numbers.First(Function(number) number > 80)

' Display the output.
MsgBox(first)

' This code produces the following output:
'
' 92

注解

如果First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)在中source未找到匹配的元素, 此方法将引发异常。The First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) method throws an exception if no matching element is found in source. 如果未找到匹配的元素, 则改为返回默认值时, FirstOrDefault请使用方法。To instead return a default value when no matching element is found, use the FirstOrDefault method.

适用于