Enumerable.DefaultIfEmpty Enumerable.DefaultIfEmpty Enumerable.DefaultIfEmpty Enumerable.DefaultIfEmpty Method

定義

傳回的項目IEnumerable<T>,或預設值單一集合,如果序列是空的。Returns the elements of an IEnumerable<T>, or a default valued singleton collection if the sequence is empty.

多載

DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)

如果序列是空的傳回單一集合中指定的序列或指定的值的項目。Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty.

DefaultIfEmpty<TSource>(IEnumerable<TSource>) DefaultIfEmpty<TSource>(IEnumerable<TSource>) DefaultIfEmpty<TSource>(IEnumerable<TSource>) DefaultIfEmpty<TSource>(IEnumerable<TSource>)

如果序列是空的傳回單一集合中指定的順序或型別參數的預設值的項目。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>(IEnumerable<TSource>, TSource) DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)

如果序列是空的傳回單一集合中指定的序列或指定的值的項目。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::Collections::Generic::IEnumerable<TSource> ^ DefaultIfEmpty(System::Collections::Generic::IEnumerable<TSource> ^ source, TSource defaultValue);
public static System.Collections.Generic.IEnumerable<TSource> DefaultIfEmpty<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, TSource defaultValue);
static member DefaultIfEmpty : seq<'Source> * 'Source -> seq<'Source>
<Extension()>
Public Function DefaultIfEmpty(Of TSource) (source As IEnumerable(Of TSource), defaultValue As TSource) As IEnumerable(Of TSource)

類型參數

TSource

source 項目的類型。The type of the elements of source.

參數

source
IEnumerable<TSource>

序列,若此序列空白,便傳回指定的值。The sequence to return the specified value for if it is empty.

defaultValue
TSource TSource TSource TSource

在序列空白時所要傳回的值。The value to return if the sequence is empty.

傳回

IEnumerable<TSource>

IEnumerable<T> ,其中包含defaultValue如果source是空的否則sourceAn IEnumerable<T> that contains defaultValue if source is empty; otherwise, source.

範例

下列程式碼範例示範如何使用DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)方法並指定預設值。The following code example demonstrates how to use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method and specify a default value. 第一個序列不是空的第二個序列是空的。The first sequence is not empty and the second sequence is empty.

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

public static void DefaultIfEmptyEx2()
{
    Pet defaultPet = new Pet { Name = "Default Pet", Age = 0 };

    List<Pet> pets1 =
        new List<Pet>{ new Pet { Name="Barley", Age=8 },
                       new Pet { Name="Boots", Age=4 },
                       new Pet { Name="Whiskers", Age=1 } };

    foreach (Pet pet in pets1.DefaultIfEmpty(defaultPet))
    {
        Console.WriteLine("Name: {0}", pet.Name);
    }

    List<Pet> pets2 = new List<Pet>();

    foreach (Pet pet in pets2.DefaultIfEmpty(defaultPet))
    {
        Console.WriteLine("\nName: {0}", pet.Name);
    }
}

/*
 This code produces the following output:

 Name: Barley
 Name: Boots
 Name: Whiskers

 Name: Default Pet
*/
Structure Pet
    Public Name As String
    Public Age As Integer
End Structure

Sub DefaultIfEmptyEx2()
    ' Create a Pet object to use as the default value.
    Dim defaultPet As New Pet With {.Name = "Default Pet", .Age = 0}

    ' Create a List of Pet objects.
    Dim pets1 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}})

    Dim output1 As New System.Text.StringBuilder
    ' Enumerate the items in the list, calling DefaultIfEmpty() 
    ' with a default value.
    For Each pet As Pet In pets1.DefaultIfEmpty(defaultPet)
        output1.AppendLine("Name: " & pet.Name)
    Next

    ' Display the output.
    MsgBox(output1.ToString())

    ' Create an empty List.
    Dim pets2 As New List(Of Pet)

    Dim output2 As New System.Text.StringBuilder
    ' Enumerate the items in the list, calling DefaultIfEmpty() 
    ' with a default value.
    For Each pet As Pet In pets2.DefaultIfEmpty(defaultPet)
        output2.AppendLine("Name: " & pet.Name)
    Next

    ' Display the output.
    MsgBox(output2.ToString())
End Sub

' This code produces the following output:
'
' Name: Barley
' Name: Boots
' Name: Whiskers
'
' Name: Default Pet

備註

實作這個方法是使用延後的執行。This method is implemented by using deferred execution. 立即傳回值是儲存,才能執行此動作的所有資訊的物件。The immediate return value is an object that stores all the information that is required to perform the action. 這個方法所表示的查詢不會執行直到列舉物件可以藉由呼叫其GetEnumerator方法直接或透過foreachVisual C# 中或For EachVisual Basic 中。The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

這個方法可以用來產生左外部聯結,當它結合GroupJoin) 方法。This method can be used to produce a left outer join when it is combined with the GroupJoin) method.

另請參閱

DefaultIfEmpty<TSource>(IEnumerable<TSource>) DefaultIfEmpty<TSource>(IEnumerable<TSource>) DefaultIfEmpty<TSource>(IEnumerable<TSource>) DefaultIfEmpty<TSource>(IEnumerable<TSource>)

如果序列是空的傳回單一集合中指定的順序或型別參數的預設值的項目。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::Collections::Generic::IEnumerable<TSource> ^ DefaultIfEmpty(System::Collections::Generic::IEnumerable<TSource> ^ source);
public static System.Collections.Generic.IEnumerable<TSource> DefaultIfEmpty<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);
static member DefaultIfEmpty : seq<'Source> -> seq<'Source>
<Extension()>
Public Function DefaultIfEmpty(Of TSource) (source As IEnumerable(Of TSource)) As IEnumerable(Of TSource)

類型參數

TSource

source 項目的類型。The type of the elements of source.

參數

source
IEnumerable<TSource>

序列,若此序列空白,便傳回預設值。The sequence to return a default value for if it is empty.

傳回

IEnumerable<TSource>

IEnumerable<T>物件,其中包含的預設值TSource如果鍵入source是空的否則sourceAn IEnumerable<T> object that contains the default value for the TSource type if source is empty; otherwise, source.

例外狀況

範例

下列程式碼範例示範如何使用DefaultIfEmpty<TSource>(IEnumerable<TSource>)提供預設值,如果來源序列是空的。The following code examples demonstrate how to use DefaultIfEmpty<TSource>(IEnumerable<TSource>) to provide a default value in case the source sequence is empty.

此範例使用非空白序列。This example uses a non-empty sequence.

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

public static void DefaultIfEmptyEx1()
{
    List<Pet> pets =
        new List<Pet>{ new Pet { Name="Barley", Age=8 },
                       new Pet { Name="Boots", Age=4 },
                       new Pet { Name="Whiskers", Age=1 } };

    foreach (Pet pet in pets.DefaultIfEmpty())
    {
        Console.WriteLine(pet.Name);
    }
}

/*
 This code produces the following output:

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

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}})

    Dim output As New System.Text.StringBuilder
    ' Iterate through the items in the List, calling DefaultIfEmpty().
    For Each pet As Pet In pets.DefaultIfEmpty()
        output.AppendLine(pet.Name)
    Next

    ' Display the output.
    MsgBox(output.ToString())
End Sub

' This code produces the following output:
'
' Barley
' Boots
' Whiskers

此範例會使用空的序列。This example uses an empty sequence.

List<int> numbers = new List<int>();

foreach (int number in numbers.DefaultIfEmpty())
{
    Console.WriteLine(number);
}

/*
 This code produces the following output:

 0
*/
' Create an empty List.
Dim numbers As New List(Of Integer)()

Dim output As New System.Text.StringBuilder
' Iterate through the items in the List, calling DefaultIfEmpty().
For Each number As Integer In numbers.DefaultIfEmpty()
    output.AppendLine(number)
Next

' Display the output.
MsgBox(output.ToString())

' This code produces the following output:
'
' 0

備註

實作這個方法是使用延後的執行。This method is implemented by using deferred execution. 立即傳回值是儲存,才能執行此動作的所有資訊的物件。The immediate return value is an object that stores all the information that is required to perform the action. 這個方法所表示的查詢不會執行直到列舉物件可以藉由呼叫其GetEnumerator方法直接或透過foreachVisual C# 中或For EachVisual Basic 中。The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

參考和可為 null 類型的預設值是nullThe default value for reference and nullable types is null.

這個方法可以用來產生左外部聯結,當它結合GroupJoin) 方法。This method can be used to produce a left outer join when it is combined with the GroupJoin) method.

另請參閱

適用於