Queryable.DefaultIfEmpty 方法

定义

返回序列中的元素,如果序列为空,则返回具有默认值的单一实例集合。Returns the elements in a sequence or a default valued singleton collection if the sequence is empty.

重载

DefaultIfEmpty<TSource>(IQueryable<TSource>)

返回指定序列中的元素;如果序列为空,则返回单一实例集合中的类型参数的默认值。Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty.

DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)

返回指定序列中的元素;如果序列为空,则返回单一实例集合中的指定值。Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty.

DefaultIfEmpty<TSource>(IQueryable<TSource>)

返回指定序列中的元素;如果序列为空,则返回单一实例集合中的类型参数的默认值。Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static System::Linq::IQueryable<TSource> ^ DefaultIfEmpty(System::Linq::IQueryable<TSource> ^ source);
public static System.Linq.IQueryable<TSource> DefaultIfEmpty<TSource> (this System.Linq.IQueryable<TSource> source);
static member DefaultIfEmpty : System.Linq.IQueryable<'Source> -> System.Linq.IQueryable<'Source>
<Extension()>
Public Function DefaultIfEmpty(Of TSource) (source As IQueryable(Of TSource)) As IQueryable(Of TSource)

类型参数

TSource

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

参数

source
IQueryable<TSource>

用于在序列为空的情况下返回默认值的 IQueryable<T>The IQueryable<T> to return a default value for if empty.

返回

IQueryable<TSource>

用于在 TSource 为空的情况下包含 default(source) 的 IQueryable<T>;否则为 sourceAn IQueryable<T> that contains default(TSource) if source is empty; otherwise, source.

异常

sourcenullsource is null.

示例

下面的代码示例演示如何使用 DefaultIfEmpty<TSource>(IQueryable<TSource>) 来提供默认值,以防源序列为空。The following code examples demonstrate how to use DefaultIfEmpty<TSource>(IQueryable<TSource>) to provide a default value in case the source sequence is empty.

class Pet
{
    public string Name { get; set; }
    public int Age { get; set; }
}

public static void DefaultIfEmptyEx1()
{
    // Create a list of Pet objects.
    List<Pet> pets =
        new List<Pet>{ new Pet { Name="Barley", Age=8 },
                       new Pet { Name="Boots", Age=4 },
                       new Pet { Name="Whiskers", Age=1 } };

    // Call DefaultIfEmtpy() on the collection that Select()
    // returns, so that if the initial list is empty, there
    // will always be at least one item in the returned array.
    string[] names =
        pets.AsQueryable()
        .Select(pet => pet.Name)
        .DefaultIfEmpty()
        .ToArray();

    string first = names[0];
    Console.WriteLine(first);
}

/*
    This code produces the following output:

    Barley
*/
Structure Pet
    Public Name As String
    Public Age As Integer
End Structure

Shared Sub DefaultIfEmptyEx1()
    ' Create a list of Pet objects.
    Dim pets As New List(Of Pet)(New Pet() { _
                        New Pet With {.Name = "Barley", .Age = 8}, _
                        New Pet With {.Name = "Boots", .Age = 4}, _
                        New Pet With {.Name = "Whiskers", .Age = 1}})

    ' Call DefaultIfEmtpy() on the collection that Select()
    ' returns, so that if the initial list is empty, there
    ' will always be at least one item in the returned array.
    Dim names() As String = pets.AsQueryable() _
        .Select(Function(Pet) Pet.Name) _
        .DefaultIfEmpty() _
        .ToArray()

    Dim first As String = names(0)
    MsgBox(first)

    ' This code produces the following output:
    '
    ' Barley

End Sub

注解

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

因执行表示调用 DefaultIfEmpty<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 DefaultIfEmpty<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. 预期的行为是,如果不为空,它将返回 sourceThe expected behavior is that it returns source if it is not empty. 否则,它将返回包含 default(TSource)IQueryable<T>Otherwise, it returns an IQueryable<T> that contains default(TSource).

DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)

返回指定序列中的元素;如果序列为空,则返回单一实例集合中的指定值。Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static System::Linq::IQueryable<TSource> ^ DefaultIfEmpty(System::Linq::IQueryable<TSource> ^ source, TSource defaultValue);
public static System.Linq.IQueryable<TSource> DefaultIfEmpty<TSource> (this System.Linq.IQueryable<TSource> source, TSource defaultValue);
static member DefaultIfEmpty : System.Linq.IQueryable<'Source> * 'Source -> System.Linq.IQueryable<'Source>
<Extension()>
Public Function DefaultIfEmpty(Of TSource) (source As IQueryable(Of TSource), defaultValue As TSource) As IQueryable(Of TSource)

类型参数

TSource

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

参数

source
IQueryable<TSource>

用于在序列为空的情况下返回指定值的 IQueryable<T>The IQueryable<T> to return the specified value for if empty.

defaultValue
TSource

序列为空时要返回的值。The value to return if the sequence is empty.

返回

IQueryable<TSource>

如果 source 为空,则为包含 defaultValueIQueryable<T>;否则为 sourceAn IQueryable<T> that contains defaultValue if source is empty; otherwise, source.

异常

sourcenullsource is null.

示例

下面的代码示例演示了在 LINQLINQ 查询中调用 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) 很有用。The following code example shows a situation in which it is useful to call DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) in a LINQLINQ query. 在此示例中,默认值将传递给 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)A default value is passed to DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) in this example.

    // Create a list of Pet objects.
    List<Pet> pets =
        new List<Pet>{ new Pet { Name="Barley", Age=8 },
                       new Pet { Name="Boots", Age=4 },
                       new Pet { Name="Whiskers", Age=1 } };

    // This query selects only those pets that are 10 or older.
    // In case there are no pets that meet that criteria, call
    // DefaultIfEmpty(). This code passes an (optional) default
    // value to DefaultIfEmpty().
    string[] oldPets =
        pets.AsQueryable()
        .Where(pet => pet.Age >= 10)
        .Select(pet => pet.Name)
        .DefaultIfEmpty("[EMPTY]")
        .ToArray();

    Console.WriteLine("First query: " + oldPets[0]);

    // This query selects only those pets that are 10 or older.
    // This code does not call DefaultIfEmpty().
    string[] oldPets2 =
        pets.AsQueryable()
        .Where(pet => pet.Age >= 10)
        .Select(pet => pet.Name)
        .ToArray();

    // There may be no elements in the array, so directly
    // accessing element 0 may throw an exception.
    try
    {
        Console.WriteLine("Second query: " + oldPets2[0]);
    }
    catch (Exception e)
    {
        Console.WriteLine("Second query: An exception was thrown: " + e.Message);
    }

    /*
        This code produces the following output:

        First query: [EMPTY]
        Second query: An exception was thrown: Index was outside the bounds of the array.
    */

' Create a list of Pet objects.
Dim pets As New List(Of Pet)(New Pet() { _
                   New Pet With {.Name = "Barley", .Age = 8}, _
                   New Pet With {.Name = "Boots", .Age = 4}, _
                   New Pet With {.Name = "Whiskers", .Age = 1}})

' This query returns pets that are 10 or older. In case there are no pets 
' that meet that criteria, call DefaultIfEmpty(). This code passes an (optional) 
' default value to DefaultIfEmpty().
Dim oldPets() As String = pets.AsQueryable() _
    .Where(Function(Pet) Pet.Age >= 10) _
    .Select(Function(Pet) Pet.Name) _
    .DefaultIfEmpty("[EMPTY]") _
    .ToArray()
Try
    MsgBox("First query: " + oldPets(0))
Catch ex As Exception
    Console.WriteLine("First query: An exception was thrown: " + ex.Message)
End Try

' This query selects only those pets that are 10 or older.
' This code does not call DefaultIfEmpty().
Dim oldPets2() As String = _
    pets.AsQueryable() _
    .Where(Function(Pet) Pet.Age >= 10) _
    .Select(Function(Pet) Pet.Name) _
    .ToArray()

' There may be no elements in the array, so directly
' accessing element 0 may throw an exception.
Try
    MsgBox("Second query: " + oldPets2(0))
Catch ex As Exception
    MsgBox("Second query: An exception was thrown: " + ex.Message)
End Try

' This code produces the following output:
'
' First(query) : [EMPTY]
' Second query: An exception was thrown: Index was outside the bounds of the array.

注解

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

因执行表示调用 DefaultIfEmpty<TSource>(IQueryable<TSource>, 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 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) depends on the implementation of the type of the source parameter. 预期的行为是,如果不为空,它将返回 sourceThe expected behavior is that it returns source if it is not empty. 否则,它将返回包含 defaultValueIQueryable<T>Otherwise, it returns an IQueryable<T> that contains defaultValue.

适用于