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);

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

类型参数

参数

返回

TSource

如果 source 为空或没有元素通过 predicate 指定的测试，则为 default(TSource)，否则为 source 中通过 predicate 指定的测试的第一个元素。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

示例

下面的代码示例演示如何通过传入谓词来使用 FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)。The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) by passing in a predicate. 在对方法的第二次调用中，数组中没有满足条件的元素。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.

注解

引用和可以为 null 的类型的默认值为 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);

static member FirstOrDefault : seq<'Source> -> 'Source

<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IEnumerable(Of TSource)) As TSource

类型参数

参数

返回

TSource

如果 source 为空，则为 default(TSource)；否则为 source 中的第一个元素。default(TSource) if source is empty; otherwise, the first element in source.

例外

示例

下面的代码示例演示如何对空数组使用 FirstOrDefault<TSource>(IEnumerable<TSource>)。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

有时，如果集合不包含任何元素，则 default(TSource) 的值不是要使用的默认值。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) 方法来指定要在集合为空时使用的默认值，而不是检查不需要的默认值的结果，然后根据需要进行更改。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. 然后，调用 First<TSource>(IEnumerable<TSource>) 获取第一个元素。Then, call First<TSource>(IEnumerable<TSource>) to obtain the first element. 下面的代码示例使用这两种方法来获取默认值1（如果数值月份的集合为空）。The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. 由于整数的默认值为0（与任何月份均不对应），因此必须将默认值指定为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. 查询执行完毕后，将检查第一个结果变量是否存在不需要的默认值。The first result variable is checked for the unwanted default value after the query has finished executing. 第二个结果变量是通过使用 DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) 指定默认值1获取的。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

注解

引用和可以为 null 的类型的默认值为 null。The default value for reference and nullable types is null.

FirstOrDefault 方法不提供指定默认值的方法。The FirstOrDefault method does not provide a way to specify a default value. 如果要指定 default(TSource)以外的默认值，请使用 "示例" 部分中所述的 DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) 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.