Enumerable.First 方法
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>) |
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>) |
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>)
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
返回指定序列中的第一个元素。The first element in the specified sequence.
例外
source 为 null。source is null.
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>) 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.
Console.WriteLine(first)
' This code produces the following output:
'
' 9
注解
如果 source 不包含任何元素，则 First<TSource>(IEnumerable<TSource>) 方法会引发异常。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，请使用 FirstOrDefault 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>)
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.
返回
- 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.
例外
source 或 predicate 为 null。source or predicate is null.
元素均不满足 predicate 中的条件。No element satisfies the condition in predicate.
- 或 --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>) 返回数组中满足条件的第一个元素。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.
Console.WriteLine(first)
' This code produces the following output:
'
' 92
注解
如果 source中找不到匹配的元素，First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) 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，请使用 FirstOrDefault 方法。To instead return a default value when no matching element is found, use the FirstOrDefault method.