Enumerable.FirstOrDefault<TSource> Method (IEnumerable<TSource>, Func<TSource, Boolean>)

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

  • 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.