Enumerable.FirstOrDefault 方法
定義
傳回序列的第一個項目;如果找不到任何項目,則傳回預設值。Returns the first element of a sequence, or a default value if no element is found.
多載
FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) |
傳回序列中符合條件的第一個元素;如果找不到這類元素,則傳回預設值。Returns the first element of the sequence that satisfies a condition or a default value if no such element is found. |
FirstOrDefault<TSource>(IEnumerable<TSource>) |
傳回序列的第一個元素;如果序列中沒有包含任何元素,則傳回預設值。Returns the first element of a sequence, or a default value if the sequence contains no elements. |
FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)
傳回序列中符合條件的第一個元素;如果找不到這類元素,則傳回預設值。Returns the first element of the 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 FirstOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, bool> ^ predicate);
public static TSource FirstOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate);
public static TSource? FirstOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate);
static member FirstOrDefault : seq<'Source> * Func<'Source, bool> -> 'Source
<Extension()>
Public Function FirstOrDefault(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.
傳回
- TSource
如果 TSource
是空的,或是沒有任何項目通過 source
所指定的測試,則為 default
(predicate
),否則為 source
中通過 predicate
指定之測試的第一個項目。default
(TSource
) if source
is empty or if no element passes the test specified by predicate
; otherwise, the first element in source
that passes the test specified by predicate
.
例外狀況
source
或 predicate
為 null
。source
or predicate
is null
.
範例
下列程式碼範例示範如何 FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) 在述詞中傳遞來使用。The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) by passing in a predicate. 在第二次呼叫方法時,陣列中沒有符合條件的元素。In the second call to the method, there is no element in the array that satisfies the condition.
string[] names = { "Hartono, Tommy", "Adams, Terry",
"Andersen, Henriette Thaulow",
"Hedlund, Magnus", "Ito, Shu" };
string firstLongName = names.FirstOrDefault(name => name.Length > 20);
Console.WriteLine("The first long name is '{0}'.", firstLongName);
string firstVeryLongName = names.FirstOrDefault(name => name.Length > 30);
Console.WriteLine(
"There is {0} name longer than 30 characters.",
string.IsNullOrEmpty(firstVeryLongName) ? "not a" : "a");
/*
This code produces the following output:
The first long name is 'Andersen, Henriette Thaulow'.
There is not a name longer than 30 characters.
*/
' Create an array of strings.
Dim names() As String =
{"Hartono, Tommy", "Adams, Terry", "Andersen, Henriette Thaulow", "Hedlund, Magnus", "Ito, Shu"}
' Select the first string in the array whose length is greater than 20.
Dim firstLongName As String =
names.FirstOrDefault(Function(name) name.Length > 20)
' Display the output.
Console.WriteLine($"The first long name is {firstLongName}")
' Select the first string in the array whose length is greater than 30,
' or a default value if there are no such strings in the array.
Dim firstVeryLongName As String =
names.FirstOrDefault(Function(name) name.Length > 30)
Dim text As String = IIf(String.IsNullOrEmpty(firstVeryLongName), "not a", "a")
Console.WriteLine($"There is {text} name longer than 30 characters.")
' This code produces the following output:
'
' The first long name is Andersen, Henriette Thaulow
'
' There is not a name longer than 30 characters.
備註
參考和可為 null 類型的預設值為 null
。The default value for reference and nullable types is null
.
適用於
FirstOrDefault<TSource>(IEnumerable<TSource>)
傳回序列的第一個元素;如果序列中沒有包含任何元素,則傳回預設值。Returns the first element of a sequence, or a default value if the sequence contains no elements.
public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
static TSource FirstOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source);
public static TSource FirstOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);
public static TSource? FirstOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);
static member FirstOrDefault : seq<'Source> -> 'Source
<Extension()>
Public Function FirstOrDefault(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
如果 TSource
是空的,則為 default
(source
),否則為 source
中的第一個項目。default
(TSource
) if source
is empty; otherwise, the first element in source
.
例外狀況
source
為 null
。source
is null
.
範例
下列程式碼範例示範如何 FirstOrDefault<TSource>(IEnumerable<TSource>) 在空陣列上使用。The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>) on an empty array.
int[] numbers = { };
int first = numbers.FirstOrDefault();
Console.WriteLine(first);
/*
This code produces the following output:
0
*/
' Create an empty array.
Dim numbers() As Integer = {}
' Select the first element in the array, or a default value
' if there are not elements in the array.
Dim first As Integer = numbers.FirstOrDefault()
' Display the output.
Console.WriteLine(first)
' This code produces the following output:
'
' 0
default(TSource)
如果集合中沒有包含任何元素,則有時候的值不是您想要使用的預設值。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) 方法來指定當集合是空的時,您要使用的預設值,而不是檢查不必要預設值的結果,然後在必要時進行變更。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. 然後,呼叫 First<TSource>(IEnumerable<TSource>) 以取得第一個元素。Then, call First<TSource>(IEnumerable<TSource>) to obtain the first element. 下列程式碼範例會使用這兩種技術來取得預設值1(如果數值月份的集合是空的)。The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. 由於整數的預設值是0,不會對應到任何月份,因此必須改為將預設值指定為1。Because the default value for an integer is 0, which does not correspond to any month, the default value must be specified as 1 instead. 查詢完成執行之後,會檢查第一個結果變數是否有不必要的預設值。The first result variable is checked for the unwanted default value after the query has finished executing. 第二個結果變數是使用 DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) 指定預設值1來取得的。The second result variable is obtained by using DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) to specify a default value of 1.
List<int> months = new List<int> { };
// Setting the default value to 1 after the query.
int firstMonth1 = months.FirstOrDefault();
if (firstMonth1 == 0)
{
firstMonth1 = 1;
}
Console.WriteLine("The value of the firstMonth1 variable is {0}", firstMonth1);
// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int firstMonth2 = months.DefaultIfEmpty(1).First();
Console.WriteLine("The value of the firstMonth2 variable is {0}", firstMonth2);
/*
This code produces the following output:
The value of the firstMonth1 variable is 1
The value of the firstMonth2 variable is 1
*/
Dim months As New List(Of Integer)(New Integer() {})
' Setting the default value to 1 after the query.
Dim firstMonth1 As Integer = months.FirstOrDefault()
If firstMonth1 = 0 Then
firstMonth1 = 1
End If
Console.WriteLine($"The value of the firstMonth1 variable is {firstMonth1}")
' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim firstMonth2 As Integer = months.DefaultIfEmpty(1).First()
Console.WriteLine($"The value of the firstMonth2 variable is {firstMonth2}")
' This code produces the following output:
'
' The value of the firstMonth1 variable is 1
' The value of the firstMonth2 variable is 1
備註
參考和可為 null 類型的預設值為 null
。The default value for reference and nullable types is null
.
FirstOrDefault方法不提供指定預設值的方法。The FirstOrDefault method does not provide a way to specify a default value. 如果您想要指定以外的預設值 default(TSource)
,請使用 DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) 範例一節中所述的方法。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.