Queryable.DefaultIfEmpty Queryable.DefaultIfEmpty Queryable.DefaultIfEmpty Queryable.DefaultIfEmpty Method

定義

傳回序列中的項目;如果序列是空的,則傳回預設值單一集合。Returns the elements in a sequence or a default valued singleton collection if the sequence is empty.

多載

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

傳回指定之序列的項目;如果序列是空的,則傳回單一集合中型別參數的預設值。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) DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)

傳回指定之序列的項目;如果序列是空的,則傳回單一集合中型別參數的預設值。Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty.

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

傳回指定之序列的項目;如果序列是空的,則傳回單一集合中型別參數的預設值。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)

類型參數

TSource

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

參數

source
IQueryable<TSource>

在空白時,要傳回預設值的 IQueryable<T>The IQueryable<T> to return a default value for if empty.

傳回

IQueryable<TSource>

如果 TSource 是空的,則為包含 default(source) 的 IQueryable<T>,否則為 sourceAn IQueryable<T> that contains default(TSource) 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>(IQueryable<TSource>)來提供預設值, 以防來源序列是空的。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

備註

方法會產生, 其代表以DefaultIfEmpty<TSource>(IQueryable<TSource>)結構化泛型方法的方式呼叫本身。 MethodCallExpression DefaultIfEmpty<TSource>(IQueryable<TSource>)The DefaultIfEmpty<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling DefaultIfEmpty<TSource>(IQueryable<TSource>) itself as a constructed generic method. 然後, 它會MethodCallExpression將傳遞CreateQuery<TElement>(Expression)至由Provider source參數IQueryProvider的屬性所表示之的方法。It then passes the MethodCallExpression to the CreateQuery<TElement>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

因執行代表呼叫DefaultIfEmpty<TSource>(IQueryable<TSource>)之運算式樹狀架構而產生的查詢行為, 取決於source參數類型的實作為。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. 預期的行為是, source如果它不是空的, 則會傳回。The expected behavior is that it returns source if it is not empty. 否則, 它會IQueryable<T>傳回包含default(TSource)的。Otherwise, it returns an IQueryable<T> that contains default(TSource).

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

傳回指定之序列的項目;如果序列是空的,則傳回單一集合中型別參數的預設值。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)

類型參數

TSource

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

參數

source
IQueryable<TSource>

在空白時,要傳回指定之值的 IQueryable<T>The IQueryable<T> to return the specified value for if 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

IQueryable<TSource>

如果 defaultValue 是空的,則為包含 sourceIQueryable<T>,否則為 sourceAn IQueryable<T> that contains defaultValue if source is empty; otherwise, source.

例外狀況

範例

下列程式碼範例顯示在DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) LINQLINQ查詢中呼叫的情況。The following code example shows a situation in which it is useful to call DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) in a LINQLINQ query. 在此範例中, 會DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)將預設值傳遞給。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.

備註

方法會產生, 其代表以DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)結構化泛型方法的方式呼叫本身。 MethodCallExpression DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)The DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method generates a MethodCallExpression that represents calling DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) itself as a constructed generic method. 然後, 它會MethodCallExpression將傳遞CreateQuery<TElement>(Expression)至由Provider source參數IQueryProvider的屬性所表示之的方法。It then passes the MethodCallExpression to the CreateQuery<TElement>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

因執行代表呼叫DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)之運算式樹狀架構而產生的查詢行為, 取決於source參數類型的實作為。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. 預期的行為是, source如果它不是空的, 則會傳回。The expected behavior is that it returns source if it is not empty. 否則, 它會IQueryable<T>傳回包含defaultValue的。Otherwise, it returns an IQueryable<T> that contains defaultValue.

適用於