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

 

Returns the first element of 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 FirstOrDefault(Of TSource) (
	source As IQueryable(Of TSource)
) As TSource

Parameters

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

The IQueryable(Of T) to return the first element of.

Return Value

Type: TSource

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

Type Parameters

TSource

The type of the elements of source.

Exception Condition
ArgumentNullException

source is null.

The FirstOrDefault(Of TSource)(IQueryable(Of TSource)) method generates a MethodCallExpression that represents calling FirstOrDefault(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 FirstOrDefault(Of TSource)(IQueryable(Of TSource)) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the first element in source, or a default value if source is empty.

The FirstOrDefault method does not provide a way to specify the default value to return if source is empty. 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 FirstOrDefault(Of TSource)(IQueryable(Of TSource)) on an empty sequence.

' Create an empty array.
Dim numbers() As Integer = {}
' Get the first item in the array, or else the 
' default value for type int, which is 0.
Dim first As Integer = numbers.AsQueryable().FirstOrDefault()

MsgBox(first)

' This code produces the following output:

' 0

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 First(Of TSource)(IQueryable(Of TSource)) to obtain the first element. The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. Because the default value for an integer is 0, which does not correspond to any 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 months As New List(Of Integer)(New Integer() {})

' Setting the default value to 1 after the query.
Dim firstMonth1 As Integer = months.AsQueryable().FirstOrDefault()
If firstMonth1 = 0 Then
    firstMonth1 = 1
End If
MsgBox(String.Format("The value of the firstMonth1 variable is {0}", firstMonth1))

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim firstMonth2 As Integer = months.AsQueryable().DefaultIfEmpty(1).First()
MsgBox(String.Format("The value of the firstMonth2 variable is {0}", firstMonth2))

' This code produces the following output:
'
' The value of the firstMonth1 variable is 1
' The value of the firstMonth2 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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