Queryable.LastOrDefault(Of TSource) Method (IQueryable(Of TSource))

 

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

Namespace:   System.Linq
Assembly:  System.Core (in System.Core.dll)

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

Parameters

source
Type: System.Linq.IQueryable(Of TSource)

An IQueryable(Of T) to return the last element of.

Return Value

Type: TSource

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

Type Parameters

TSource

The type of the elements of source.

Exception Condition
ArgumentNullException

source is null.

The LastOrDefault(Of TSource)(IQueryable(Of TSource)) method generates a MethodCallExpression that represents calling LastOrDefault(Of TSource)(IQueryable(Of TSource)) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute(Of 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(Of TSource)(IQueryable(Of 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(Of TSource)(IQueryable(Of TSource), TSource) method as described in the Example section.

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

' 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(Of TSource)(IQueryable(Of TSource), TSource) method to specify the default value that you want to use if the collection is empty. Then, call Last(Of TSource)(IQueryable(Of 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(Of TSource)(IQueryable(Of TSource), TSource) to specify a default value of 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

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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