Queryable.FirstOrDefault 方法

定义

返回序列中的第一个元素;如果未找到该元素,则返回默认值。Returns the first element of a sequence, or a default value if no element is found.

重载

FirstOrDefault<TSource>(IQueryable<TSource>)

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.

FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)

返回序列中满足指定条件的第一个元素,如果未找到这样的元素,则返回默认值。Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found.

FirstOrDefault<TSource>(IQueryable<TSource>)

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::Linq::IQueryable<TSource> ^ source);
public static TSource FirstOrDefault<TSource> (this System.Linq.IQueryable<TSource> source);
static member FirstOrDefault : System.Linq.IQueryable<'Source> -> 'Source
<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IQueryable(Of TSource)) As TSource

类型参数

TSource

source 的元素类型。The type of the elements of source.

参数

source
IQueryable<TSource>

要返回其第一个元素的 IQueryable<T>The IQueryable<T> to return the first element of.

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.

异常

sourcenullsource is null.

示例

下面的代码示例演示如何对空序列使用 FirstOrDefault<TSource>(IQueryable<TSource>)The following code example demonstrates how to use FirstOrDefault<TSource>(IQueryable<TSource>) on an empty sequence.

// Create an empty array.
int[] numbers = { };
// Get the first item in the array, or else the 
// default value for type int (0).
int first = numbers.AsQueryable().FirstOrDefault();

Console.WriteLine(first);

/*
    This code produces the following output:

    0
*/

' Create an empty array.
Dim numbers() As Integer = {}
' Get the first item in the array, or else the 
' default value for type int, which is 0.
Dim first As Integer = numbers.AsQueryable().FirstOrDefault()

MsgBox(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>(IQueryable<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>(IQueryable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. 然后,调用 First<TSource>(IQueryable<TSource>) 获取第一个元素。Then, call First<TSource>(IQueryable<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. 查询完成后,将检查第一个结果变量中是否有不需要的默认值。The first result variable is checked for the unwanted default value after the query is completed. 第二个结果变量是通过调用 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) 指定默认值1获取的。The second result variable is obtained by calling DefaultIfEmpty<TSource>(IQueryable<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.AsQueryable().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.AsQueryable().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.AsQueryable().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.AsQueryable().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

FirstOrDefault<TSource>(IQueryable<TSource>) 方法生成一个 MethodCallExpression,它表示作为构造的泛型方法调用 FirstOrDefault<TSource>(IQueryable<TSource>) 本身。The FirstOrDefault<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling FirstOrDefault<TSource>(IQueryable<TSource>) itself as a constructed generic method. 然后,它将 MethodCallExpression 传递到 IQueryProviderExecute<TResult>(Expression) 方法(由 source 参数的 Provider 属性表示)。It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

因执行表示调用 FirstOrDefault<TSource>(IQueryable<TSource>) 的表达式树而发生的查询行为取决于 source 参数类型的实现。The query behavior that occurs as a result of executing an expression tree that represents calling FirstOrDefault<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. 预期的行为是返回 source中的第一个元素,如果 source 为空,则返回默认值。The expected behavior is that it returns the first element in source, or a default value if source is empty.

source 为空时,FirstOrDefault 方法不提供指定要返回的默认值的方法。The FirstOrDefault method does not provide a way to specify the default value to return if source is empty. 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>(IQueryable<TSource>, TSource) 方法。If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method as described in the Example section.

FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)

返回序列中满足指定条件的第一个元素,如果未找到这样的元素,则返回默认值。Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, bool> ^> ^ predicate);
public static TSource FirstOrDefault<TSource> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,bool>> predicate);
static member FirstOrDefault : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, bool>> -> 'Source
<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IQueryable(Of TSource), predicate As Expression(Of Func(Of TSource, Boolean))) As TSource

类型参数

TSource

source 的元素类型。The type of the elements of source.

参数

source
IQueryable<TSource>

要从中返回元素的 IQueryable<T>An IQueryable<T> to return an element from.

predicate
Expression<Func<TSource,Boolean>>

用于测试每个元素是否满足条件的函数。A function to test each element for a condition.

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.

异常

sourcepredicatenullsource or predicate is null.

示例

下面的代码示例演示如何通过传入谓词来使用 FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)The following code example demonstrates how to use FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) by passing in a predicate. 在第二个查询中,序列中没有满足条件的元素。In the second query, there is no element in the sequence that satisfies the condition.

string[] names = { "Hartono, Tommy", "Adams, Terry", 
                     "Andersen, Henriette Thaulow", 
                     "Hedlund, Magnus", "Ito, Shu" };

// Get the first string in the array that is longer
// than 20 characters, or the default value for type
// string (null) if none exists.
string firstLongName =
    names.AsQueryable().FirstOrDefault(name => name.Length > 20);

Console.WriteLine("The first long name is '{0}'.", firstLongName);

// Get the first string in the array that is longer
// than 30 characters, or the default value for type
// string (null) if none exists.
string firstVeryLongName =
    names.AsQueryable().FirstOrDefault(name => name.Length > 30);

Console.WriteLine(
    "There is {0} name that is 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 that is longer than 30 characters.
*/

Dim names() As String = {"Hartono, Tommy", "Adams, Terry", _
                     "Andersen, Henriette Thaulow", _
                     "Hedlund, Magnus", "Ito, Shu"}

' Get the first string in the array that is longer
' than 20 characters, or the default value for type
' string (null) if none exists.
Dim firstLongName As String = _
            names.AsQueryable().FirstOrDefault(Function(name) name.Length > 20)

MsgBox(String.Format("The first long name is '{0}'.", firstLongName))

' Get the first string in the array that is longer
' than 30 characters, or the default value for type
' string (null) if none exists.
Dim firstVeryLongName As String = _
    names.AsQueryable().FirstOrDefault(Function(name) name.Length > 30)

MsgBox(String.Format( _
    "There is {0} name that is longer than 30 characters.", _
    IIf(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 that is longer than 30 characters.

注解

此方法至少具有一个类型 Expression<TDelegate> 的参数,其类型参数为 Func<T,TResult> 类型之一。This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. 对于这些参数,可以传入 lambda 表达式,并将其编译为 Expression<TDelegate>For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) 方法生成一个 MethodCallExpression,它表示作为构造的泛型方法调用 FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) 本身。The FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) method generates a MethodCallExpression that represents calling FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) itself as a constructed generic method. 然后,它将 MethodCallExpression 传递到 IQueryProviderExecute<TResult>(Expression) 方法(由 source 参数的 Provider 属性表示)。It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

因执行表示调用 FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) 的表达式树而发生的查询行为取决于 source 参数类型的实现。The query behavior that occurs as a result of executing an expression tree that represents calling FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) depends on the implementation of the type of the source parameter. 预期的行为是返回满足 predicate中条件的 source 中的第一个元素; 如果没有元素满足该条件,则返回默认值。The expected behavior is that it returns the first element in source that satisfies the condition in predicate, or a default value if no element satisfies the condition.

适用于