Enumerable.FirstOrDefault(TSource) Method (IEnumerable(TSource)) (System.Linq)

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Microsoft Visual Studio 2008/.NET Framework 3.5

Other versions are also available for the following:

.NET Framework Class Library

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)

Syntax

Visual Basic (Declaration)

<ExtensionAttribute> _
Public Shared Function FirstOrDefault(Of TSource) ( _
    source As IEnumerable(Of TSource) _
) As TSource

Visual Basic (Usage)

Dim source As IEnumerable(Of TSource)
Dim returnValue As TSource

returnValue = source.FirstOrDefault()

public static TSource FirstOrDefault<TSource>(
    this IEnumerable<TSource> source
)

Visual C++

[ExtensionAttribute]
public:
generic<typename TSource>
static TSource FirstOrDefault(
    IEnumerable<TSource>^ source
)

JScript

JScript does not support generic types or methods.

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).

Exceptions

Exception	Condition
ArgumentNullException	source is nullNothingnullptra null reference (Nothing in Visual Basic).

Remarks

The default value for reference and nullable types is nullNothingnullptra null reference (Nothing in Visual Basic).

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.

Examples

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

Visual Basic

' 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

int[] numbers = { };
int first = numbers.FirstOrDefault();
Console.WriteLine(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.

Visual Basic

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

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

// Setting the default value to 1 after the query.
int firstMonth1 = months.FirstOrDefault();
if (firstMonth1 == 0)
{
    firstMonth1 = 1;
}
Console.WriteLine("The value of the firstMonth1 variable is {0}", firstMonth1);

// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int firstMonth2 = months.DefaultIfEmpty(1).First();
Console.WriteLine("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
*/

Platforms

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Version Information

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0

Reference

Enumerable Class

Enumerable Members

FirstOrDefault Overload

System.Linq Namespace