Enumerable.FirstOrDefault 方法

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

參數

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

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.

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

參數

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

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.