Queryable.LastOrDefault Method

Definition

Namespace:

System.Linq

Assembly:

System.Linq.Queryable.dll

Returns the last element of a sequence, or a default value if no element is found.

Overloads

LastOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>, TSource)	Returns the last element of a sequence that satisfies a condition or a default value if no such element is found.
LastOrDefault<TSource>(IQueryable<TSource>, TSource)	Returns the last element of a sequence, or a default value if the sequence contains no elements.
LastOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)	Returns the last element of a sequence that satisfies a condition or a default value if no such element is found.
LastOrDefault<TSource>(IQueryable<TSource>)	Returns the last element in a sequence, or a default value if the sequence contains no elements.

LastOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>, TSource)

Source:

Queryable.cs

Returns the last element of a sequence that satisfies a condition or a default value if no such element is found.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource LastOrDefault(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, bool> ^> ^ predicate, TSource defaultValue);

public static TSource LastOrDefault<TSource> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,bool>> predicate, TSource defaultValue);

static member LastOrDefault : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, bool>> * 'Source -> 'Source

<Extension()>
Public Function LastOrDefault(Of TSource) (source As IQueryable(Of TSource), predicate As Expression(Of Func(Of TSource, Boolean)), defaultValue As TSource) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

An IEnumerable<T> to return an element from.

predicate

Expression<Func<TSource,Boolean>>

A function to test each element for a condition.

defaultValue

TSource

The default value to return if the sequence is empty.

Returns

TSource

defaultValue if the sequence is empty or if no elements pass the test in the predicate function; otherwise, the last element that passes the test in the predicate function.

Exceptions

ArgumentNullException

source or predicate is null.

LastOrDefault<TSource>(IQueryable<TSource>, TSource)

Source:

Queryable.cs

Returns the last element of a sequence, or a default value if the sequence contains no elements.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource LastOrDefault(System::Linq::IQueryable<TSource> ^ source, TSource defaultValue);

public static TSource LastOrDefault<TSource> (this System.Linq.IQueryable<TSource> source, TSource defaultValue);

static member LastOrDefault : System.Linq.IQueryable<'Source> * 'Source -> 'Source

<Extension()>
Public Function LastOrDefault(Of TSource) (source As IQueryable(Of TSource), defaultValue As TSource) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

An IEnumerable<T> to return the last element of.

defaultValue

TSource

The default value to return if the sequence is empty.

Returns

TSource

defaultValue if the source sequence is empty; otherwise, the last element in the IEnumerable<T>.

Exceptions

ArgumentNullException

source is null.

LastOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)

Source:

Queryable.cs

Returns the last element of a sequence that satisfies a condition or a default value if no such element is found.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource LastOrDefault(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, bool> ^> ^ predicate);

public static TSource? LastOrDefault<TSource> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,bool>> predicate);

static member LastOrDefault : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, bool>> -> 'Source

<Extension()>
Public Function LastOrDefault(Of TSource) (source As IQueryable(Of TSource), predicate As Expression(Of Func(Of TSource, Boolean))) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

An IQueryable<T> to return an element from.

predicate

Expression<Func<TSource,Boolean>>

A function to test each element for a condition.

Returns

TSource

default(TSource) if source is empty or if no elements pass the test in the predicate function; otherwise, the last element of source that passes the test in the predicate function.

Exceptions

ArgumentNullException

source or predicate is null.

Examples

The following code example demonstrates how to use LastOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) by passing in a predicate. In the second call to the method, there is no element in the sequence that satisfies the condition.

double[] numbers = { 49.6, 52.3, 51.0, 49.4, 50.2, 48.3 };

// Get the last number in the array that rounds to 50.0,
// or else the default value for type double (0.0).
double last50 =
    numbers.AsQueryable().LastOrDefault(n => Math.Round(n) == 50.0);

Console.WriteLine("The last number that rounds to 50 is {0}.", last50);

// Get the last number in the array that rounds to 40.0,
// or else the default value for type double (0.0).
double last40 =
    numbers.AsQueryable().LastOrDefault(n => Math.Round(n) == 40.0);

Console.WriteLine(
    "The last number that rounds to 40 is {0}.",
    last40 == 0.0 ? "[DOES NOT EXIST]" : last40.ToString());

/*
    This code produces the following output:

    The last number that rounds to 50 is 50.2.
    The last number that rounds to 40 is [DOES NOT EXIST].
*/

Dim numbers() As Double = {49.6, 52.3, 51.0, 49.4, 50.2, 48.3}

' Get the last number in the array that rounds to 50.0,
' or else the default value for type double (0.0).
Dim last50 As Double = _
     numbers.AsQueryable().LastOrDefault(Function(n) Math.Round(n) = 50.0)

MsgBox(String.Format("The last number that rounds to 50 is {0}.", last50))

' Get the last number in the array that rounds to 40.0,
' or else the default value for type double (0.0).
Dim last40 As Double = _
    numbers.AsQueryable().LastOrDefault(Function(n) Math.Round(n) = 40.0)

MsgBox(String.Format("The last number that rounds to 40 is {0}.", _
    IIf(last40 = 0.0, "[DOES NOT EXIST]", last40.ToString())))

'This code produces the following output:

'The last number that rounds to 50 is 50.2.
'The last number that rounds to 40 is [DOES NOT EXIST].

Remarks

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The LastOrDefault<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling LastOrDefault<TSource>(IQueryable<TSource>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(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 LastOrDefault<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the last element in source that satisfies the condition specified by predicate. It returns a default value if there is no such element in source.

LastOrDefault<TSource>(IQueryable<TSource>)

Source:

Queryable.cs

Returns the last element in a sequence, or a default value if the sequence contains no elements.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource LastOrDefault(System::Linq::IQueryable<TSource> ^ source);

public static TSource? LastOrDefault<TSource> (this System.Linq.IQueryable<TSource> source);

static member LastOrDefault : System.Linq.IQueryable<'Source> -> 'Source

<Extension()>
Public Function LastOrDefault(Of TSource) (source As IQueryable(Of TSource)) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

An IQueryable<T> to return the last element of.

Returns

TSource

default(TSource) if source is empty; otherwise, the last element in source.

Exceptions

ArgumentNullException

source is null.

Examples

The following code example demonstrates how to use LastOrDefault<TSource>(IQueryable<TSource>) on an empty array.

// Create an empty array.
string[] fruits = { };

// Get the last item in the array, or else the default
// value for type string (null).
string last = fruits.AsQueryable().LastOrDefault();

Console.WriteLine(
    String.IsNullOrEmpty(last) ? "[STRING IS NULL OR EMPTY]" : last);

/*
    This code produces the following output:

    [STRING IS NULL OR EMPTY]
*/

' Create an empty array.
Dim fruits() As String = {}

' Get the last item in the array, or else the default
' value for type string (null).
Dim last As String = fruits.AsQueryable().LastOrDefault()

MsgBox(IIf(String.IsNullOrEmpty(last), "[STRING IS NULL OR EMPTY]", last))

' This code produces the following output:
' [STRING IS NULL OR EMPTY]

Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. Instead of checking the result for the unwanted default value and then changing it if necessary, you can use the DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. Then, call Last<TSource>(IQueryable<TSource>) to obtain the last element. The following code example uses both techniques to obtain a default value of 1 if a collection of numeric days of the month is empty. Because the default value for an integer is 0, which does not correspond to any day of the month, the default value must be specified as 1 instead. The first result variable is checked for the unwanted default value after the query is completed. The second result variable is obtained by calling DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) to specify a default value of 1.

List<int> daysOfMonth = new List<int> { };

// Setting the default value to 1 after the query.
int lastDay1 = daysOfMonth.AsQueryable().LastOrDefault();
if (lastDay1 == 0)
{
    lastDay1 = 1;
}
Console.WriteLine("The value of the lastDay1 variable is {0}", lastDay1);

// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int lastDay2 = daysOfMonth.AsQueryable().DefaultIfEmpty(1).Last();
Console.WriteLine("The value of the lastDay2 variable is {0}", lastDay2);

/*
 This code produces the following output:

 The value of the lastDay1 variable is 1
 The value of the lastDay2 variable is 1
*/

Dim daysOfMonth As New List(Of Integer)(New Integer() {})

' Setting the default value to 1 after the query.
Dim lastDay1 As Integer = daysOfMonth.AsQueryable().LastOrDefault()
If lastDay1 = 0 Then
    lastDay1 = 1
End If
MsgBox(String.Format("The value of the lastDay1 variable is {0}", lastDay1))

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim lastDay2 As Integer = daysOfMonth.AsQueryable().DefaultIfEmpty(1).Last()
MsgBox(String.Format("The value of the lastDay2 variable is {0}", lastDay2))

' This code produces the following output:
'
' The value of the lastDay1 variable is 1
' The value of the lastDay2 variable is 1

Remarks

The LastOrDefault<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling LastOrDefault<TSource>(IQueryable<TSource>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(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 LastOrDefault<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the last element in source, or a default value if source is empty.

The LastOrDefault method does not provide a way to specify a default value. If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method as described in the Example section.