Enumerable.FirstOrDefault<TSource> Method (IEnumerable<TSource>, Func<TSource, Boolean>)
Microsoft Silverlight will reach end of support after October 2021. Learn more.
Returns the first element of the sequence that satisfies a condition or a default value if no such element is found.
Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)
Syntax
'Declaration
<ExtensionAttribute> _
Public Shared Function FirstOrDefault(Of TSource) ( _
source As IEnumerable(Of TSource), _
predicate As Func(Of TSource, Boolean) _
) As TSource
public static TSource FirstOrDefault<TSource>(
this IEnumerable<TSource> source,
Func<TSource, bool> predicate
)
Type Parameters
- TSource
The type of the elements of source.
Parameters
- source
Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> to return an element from.
- predicate
Type: System.Func<TSource, Boolean>
A function to test each element for a condition.
Return Value
Type: TSource
default (TSource) if source is empty or if no element passes the test specified by predicate; otherwise, the first element in source that passes the test specified by predicate.
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 or predicate 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).
Examples
The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>) by passing in a predicate. In the second call to the method, there is no element in the array that satisfies the condition.
' Create an array of strings.
Dim names() As String = _
{"Hartono, Tommy", "Adams, Terry", "Andersen, Henriette Thaulow", "Hedlund, Magnus", "Ito, Shu"}
' Select the first string in the array whose length is greater than 20.
Dim firstLongName As String = _
names.FirstOrDefault(Function(name) name.Length > 20)
' Display the output.
outputBlock.Text &= "The first long name is " & firstLongName & vbCrLf
' Select the first string in the array whose length is greater than 30,
' or a default value if there are no such strings in the array.
Dim firstVeryLongName As String = _
names.FirstOrDefault(Function(name) name.Length > 30)
Dim text As String = IIf(String.IsNullOrEmpty(firstVeryLongName), "not a", "a")
outputBlock.Text &= "There is " & text & " name longer than 30 characters." & vbCrLf
' This code produces the following output:
'
' The first long name is Andersen, Henriette Thaulow
'
' There is not a name longer than 30 characters.
string[] names = { "Hartono, Tommy", "Adams, Terry",
"Andersen, Henriette Thaulow",
"Hedlund, Magnus", "Ito, Shu" };
string firstLongName = names.FirstOrDefault(name => name.Length > 20);
outputBlock.Text += String.Format("The first long name is '{0}'.", firstLongName) + "\n";
string firstVeryLongName = names.FirstOrDefault(name => name.Length > 30);
outputBlock.Text += String.Format(
"There is {0} name longer than 30 characters.",
string.IsNullOrEmpty(firstVeryLongName) ? "not a" : "a") + "\n";
/*
This code produces the following output:
The first long name is 'Andersen, Henriette Thaulow'.
There is not a name longer than 30 characters.
*/
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.