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

Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function SingleOrDefault(Of TSource) ( _
    source As IEnumerable(Of TSource), _
    predicate As Func(Of TSource, Boolean) _
) As TSource
public static TSource SingleOrDefault<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 an element for a condition.

Return Value

Type: TSource
The single element of the input sequence that satisfies the condition, or default(TSource) if no such element is found.

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 SingleOrDefault<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>) to select the only element of an array that satisfies a condition.

' Create an array of strings.
Dim fruits() As String = _
    {"apple", "banana", "mango", "orange", "passionfruit", "grape"}

' Get the single item in the array whose length is > 10.
Dim fruit1 As String = _
    fruits.SingleOrDefault(Function(fruit) fruit.Length > 10)

' Display the result.
outputBlock.Text &= "First array: " & fruit1 & vbCrLf
      string[] fruits = { "apple", "banana", "mango", 
                               "orange", "passionfruit", "grape" };

      string fruit1 = fruits.SingleOrDefault(fruit => fruit.Length > 10);

      outputBlock.Text += fruit1 + "\n";

      /*
       This code produces the following output:

       passionfruit
      */

The following code example demonstrates that SingleOrDefault<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>) returns a default value when the sequence does not contain exactly one element that satisfies the condition.

      ' Get the single item in the array whose length is > 15.
      Dim fruit2 As String = _
          fruits.SingleOrDefault(Function(fruit) fruit.Length > 15)

      ' Display the result.
      Dim output As String = _
          IIf(String.IsNullOrEmpty(fruit2), "No single item found", fruit2)
      outputBlock.Text &= "Second array: " & output & vbCrLf

      ' This code produces the following output:
      '
      ' First array: passionfruit
      ' Second array: No single item found

      string fruit2 =
          fruits.SingleOrDefault(fruit => fruit.Length > 15);

      outputBlock.Text +=
          String.IsNullOrEmpty(fruit2) ? "No such string!" : fruit2 + "\n";

      /*
       This code produces the following output:

       No such string!
      */

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.