Export (0) Print
Expand All
0 out of 2 rated this helpful - Rate this topic

Enumerable.FirstOrDefault(Of TSource) Method (IEnumerable(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)
'Declaration
<ExtensionAttribute> _
Public Shared Function FirstOrDefault(Of TSource) ( _
	source As IEnumerable(Of TSource) _
) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable(Of TSource)
The IEnumerable(Of T) to return the first element of.

Return Value

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

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable(Of TSource). When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).
ExceptionCondition
ArgumentNullException

source is Nothing.

The default value for reference and nullable types is Nothing.

The FirstOrDefault 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)(IEnumerable(Of TSource), TSource) method as described in the Example section.

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


        ' Create an empty array.
        Dim numbers() As Integer = {}

        ' Select the first element in the array, or a default value
        ' if there are not elements in the array.
        Dim first As Integer = numbers.FirstOrDefault()

        ' Display the output.
        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)(IEnumerable(Of TSource), TSource) method to specify the default value that you want to use if the collection is empty. Then, call First(Of TSource)(IEnumerable(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 has finished executing. The second result variable is obtained by using DefaultIfEmpty(Of TSource)(IEnumerable(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.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.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



.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
Did you find this helpful?
(1500 characters remaining)
Thank you for your feedback

Community Additions

ADD
Show:
© 2014 Microsoft. All rights reserved.