Enumerable.FirstOrDefault<TSource> Method (IEnumerable<TSource>)
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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
'Declaration
<ExtensionAttribute> _
Public Shared Function FirstOrDefault(Of TSource) ( _
source As IEnumerable(Of TSource) _
) As TSource
public static TSource FirstOrDefault<TSource>(
this IEnumerable<TSource> source
)
Type Parameters
- TSource
The type of the elements of source.
Parameters
- source
Type: System.Collections.Generic.IEnumerable<TSource>
The IEnumerable<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<TSource>. When you use instance method syntax to call this method, omit the first parameter.
Exceptions
| Exception | Condition |
|---|---|
| ArgumentNullException | source is nulla null reference (Nothing in Visual Basic). |
Remarks
The default value for reference and nullable types is nulla 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<TSource>(IEnumerable<TSource>, TSource) method as described in the Example section.
Examples
The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<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.
outputBlock.Text &= first & vbCrLf
' This code produces the following output:
'
' 0
int[] numbers = { };
int first = numbers.FirstOrDefault();
outputBlock.Text += first + "\n";
/*
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<TSource>(IEnumerable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. Then, call First<TSource>(IEnumerable<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<TSource>(IEnumerable<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
outputBlock.Text &= String.Format("The value of the firstMonth1 variable is {0}", firstMonth1) _
& vbCrLf
' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim firstMonth2 As Integer = months.DefaultIfEmpty(1).First()
outputBlock.Text &= String.Format("The value of the firstMonth2 variable is {0}", firstMonth2) & _
vbCrLf
' 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;
}
outputBlock.Text += String.Format("The value of the firstMonth1 variable is {0}", firstMonth1) + "\n";
// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int firstMonth2 = months.DefaultIfEmpty(1).First();
outputBlock.Text += String.Format("The value of the firstMonth2 variable is {0}", firstMonth2) + "\n";
/*
This code produces the following output:
The value of the firstMonth1 variable is 1
The value of the firstMonth2 variable is 1
*/
Version Information
Silverlight
Supported in: 5, 4, 3
Silverlight for Windows Phone
Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0
XNA Framework
Supported in: Xbox 360, Windows Phone OS 7.0
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.