Queryable.DefaultIfEmpty Method
Definition
Important
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Returns the elements in a sequence or a default valued singleton collection if the sequence is empty.
Overloads
| DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) |
Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty. |
| DefaultIfEmpty<TSource>(IQueryable<TSource>) |
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)
- Source:
- Queryable.cs
- Source:
- Queryable.cs
- Source:
- Queryable.cs
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)Type Parameters
- TSource
The type of the elements of source.
Parameters
- source
- IQueryable<TSource>
The IQueryable<T> to return the specified value for if empty.
- defaultValue
- TSource
The value to return if the sequence is empty.
Returns
An IQueryable<T> that contains defaultValue if source is empty; otherwise, source.
Exceptions
source is null.
Examples
The following code example shows a situation in which it is useful to call DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) in a LINQ query. 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.
Remarks
The DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method generates a MethodCallExpression that represents calling DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery<TElement>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.
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. The expected behavior is that it returns source if it is not empty. Otherwise, it returns an IQueryable<T> that contains defaultValue.
Applies to
DefaultIfEmpty<TSource>(IQueryable<TSource>)
- Source:
- Queryable.cs
- Source:
- Queryable.cs
- Source:
- Queryable.cs
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);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)Type Parameters
- TSource
The type of the elements of source.
Parameters
- source
- IQueryable<TSource>
The IQueryable<T> to return a default value for if empty.
Returns
An IQueryable<T> that contains default(TSource) if source is empty; otherwise, source.
Exceptions
source is null.
Examples
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
Remarks
The DefaultIfEmpty<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling DefaultIfEmpty<TSource>(IQueryable<TSource>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery<TElement>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.
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. The expected behavior is that it returns source if it is not empty. Otherwise, it returns an IQueryable<T> that contains default(TSource).
