Enumerable.First 方法

定義

命名空間:
System.Linq
組件:
System.Linq.dll
組件:
System.Core.dll
組件:
netstandard.dll

傳回序列的第一個項目。Returns the first element of a sequence.

多載

First<TSource>(IEnumerable<TSource>)

傳回序列的第一個項目。Returns the first element of a sequence.
First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)

傳回序列中符合指定條件的第一個元素。Returns the first element in a sequence that satisfies a specified condition.

First<TSource>(IEnumerable<TSource>)

傳回序列的第一個項目。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

類型參數

TSource

source 項目的類型。The type of the elements of source.

參數

source
IEnumerable<TSource>

要傳回第一個項目的 IEnumerable<T>。The IEnumerable<T> to return the first element of.

傳回

TSource

指定之序列中的第一個項目。The first element in the specified sequence.

例外狀況

ArgumentNullException

source 為 null。source is null.

InvalidOperationException

來源序列為空。The source sequence is empty.

範例

下列程式碼範例示範如何使用傳回 First<TSource>(IEnumerable<TSource>) 陣列的第一個元素。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

備註

First<TSource>(IEnumerable<TSource>)如果 source 沒有包含任何元素，方法會擲回例外狀況。The First<TSource>(IEnumerable<TSource>) method throws an exception if source contains no elements. 當來源序列是空的時，若要改為傳回預設值，請使用 FirstOrDefault 方法。To instead return a default value when the source sequence is empty, use the FirstOrDefault method.

適用於

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

傳回序列中符合指定條件的第一個元素。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

類型參數

TSource

source 項目的類型。The type of the elements of source.

參數

source
IEnumerable<TSource>

傳回項目的 IEnumerable<T>。An IEnumerable<T> to return an element from.

predicate
Func<TSource,Boolean>

用來測試每個項目是否符合條件的函式。A function to test each element for a condition.

傳回

TSource

序列中通過指定之述詞函式所做測試的第一個項目。The first element in the sequence that passes the test in the specified predicate function.

例外狀況

ArgumentNullException

source 或 predicate 為 null。source or predicate is null.

InvalidOperationException

沒有任何項目符合 predicate 的條件。No element satisfies the condition in predicate.

-或--or- 來源序列為空。The source sequence is empty.

範例

下列程式碼範例示範如何使用傳回 First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) 符合條件之陣列的第一個元素。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

備註

First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)如果在中找不到相符的元素，方法會擲回例外狀況 source 。The First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) method throws an exception if no matching element is found in source. 若要改為在找不到相符的元素時傳回預設值，請使用 FirstOrDefault 方法。To instead return a default value when no matching element is found, use the FirstOrDefault method.

適用於