Queryable.DefaultIfEmpty 方法

定義

命名空間:

System.Linq

組件:

netstandard.dll, System.Linq.Queryable.dll

回傳序列中的元素，或若序列為空，則回傳預設值的單例集合。

多載

名稱	Description
DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)	如果序列是空的，則傳回指定序列的專案或單一集合中的指定值。
DefaultIfEmpty<TSource>(IQueryable<TSource>)	如果序列是空的，則傳回指定序列的專案或單一集合中型別參數的預設值。

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

來源:

Queryable.cs

如果序列是空的，則傳回指定序列的專案或單一集合中的指定值。

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

元素 source的類型。

參數

source

IQueryable<TSource>

若為空，則 IQueryable<T> 回傳指定的值。

defaultValue

TSource

如果序列為空，則回傳的值。

傳回

IQueryable<TSource>

IQueryable<T>一個包含若defaultValue為source空則為空;否則，。 source

例外狀況

ArgumentNullException

source 為 null。

範例

以下程式碼範例展示了一種在呼叫 LINQ 查詢時非常有用 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) 的情況。 此範例中傳遞 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) 一個預設值。

// 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

該 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) 方法產生的 是 MethodCallExpression ，代表 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) 呼叫本身為一個構造化的泛型方法。 接著將 傳遞MethodCallExpression給CreateQuery<TElement>(Expression)IQueryProvider由參數Provider性質source所表示的方法。

執行代表呼叫 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) 的表達式樹所產生的查詢行為，取決於參數型別 source 的實作。 預期行為是如果它不是空的，它會回傳 source 。 否則，它會回傳包含 IQueryable<T>defaultValue的 。

DefaultIfEmpty<TSource>(IQueryable<TSource>)

來源:

Queryable.cs

如果序列是空的，則傳回指定序列的專案或單一集合中型別參數的預設值。

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

元素 source的類型。

參數

source

IQueryable<TSource>

如果是空，則 IQueryable<T> 回傳預設值。

傳回

IQueryable<TSource>

一個包含（TSourcedefault）的若IQueryable<T>為source空;否則，。 source

例外狀況

ArgumentNullException

source 為 null。

範例

以下程式碼範例示範如何在 DefaultIfEmpty<TSource>(IQueryable<TSource>) 原始序列為空時提供預設值。

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

該 DefaultIfEmpty<TSource>(IQueryable<TSource>) 方法產生的 是 MethodCallExpression ，代表 DefaultIfEmpty<TSource>(IQueryable<TSource>) 呼叫本身為一個構造化的泛型方法。 接著將 傳遞MethodCallExpression給CreateQuery<TElement>(Expression)IQueryProvider由參數Provider性質source所表示的方法。

執行代表呼叫 DefaultIfEmpty<TSource>(IQueryable<TSource>) 的表達式樹所產生的查詢行為，取決於參數型別 source 的實作。 預期行為是如果它不是空的，它會回傳 source 。 否則，它會回傳包含 IQueryable<T>default(TSource)的 。