Enumerable.FirstOrDefault Enumerable.FirstOrDefault Enumerable.FirstOrDefault Enumerable.FirstOrDefault Method

返回序列中满足条件的第一个元素；如果未找到这样的元素，则返回默认值。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);

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

异常

示例

下面的代码示例演示如何通过传入FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)谓词来使用。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.
MsgBox("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")

MsgBox("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

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

返回序列中的第一个元素；如果序列中不包含任何元素，则返回默认值。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);

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

异常

示例

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

' This code produces the following output:
'
' 0

有时, 如果集合default(TSource)不包含任何元素, 则的值将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, 并在必要时对其进行更改。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>)以获取第一个元素。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. 第二个结果变量通过使用DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)来指定默认值1。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
MsgBox(String.Format("The value of the firstMonth1 variable is {0}", firstMonth1))

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim firstMonth2 As Integer = months.DefaultIfEmpty(1).First()
MsgBox(String.Format("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

注解

引用类型和可以为 null 的类型的null默认值为。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. 如果要指定之外的默认值default(TSource), 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.