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Enumerable.Single(Of TSource) Method (IEnumerable(Of TSource), Func(Of TSource, Boolean))

Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists.

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

'Declaration
<ExtensionAttribute> _
Public Shared Function Single(Of TSource) ( _
	source As IEnumerable(Of TSource), _
	predicate As Func(Of TSource, Boolean) _
) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable(Of TSource)
An IEnumerable(Of T) to return a single element from.
predicate
Type: System.Func(Of TSource, Boolean)
A function to test an element for a condition.

Return Value

Type: TSource
The single element of the input sequence that satisfies a condition.

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.

ExceptionCondition
ArgumentNullException

source or predicate is Nothing.

InvalidOperationException

No element satisfies the condition in predicate.

-or-

More than one element satisfies the condition in predicate.

-or-

The source sequence is empty.

The Single(Of TSource)(IEnumerable(Of TSource), Func(Of TSource, Boolean)) method throws an exception if the input sequence contains no matching element. To instead return Nothing when no matching element is found, use SingleOrDefault.

The following code example demonstrates how to use Single(Of TSource)(IEnumerable(Of TSource), Func(Of 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 greater than 10.
Dim result As String = _
    fruits.Single(Function(fruit) fruit.Length > 10)

' Display the result.
outputBlock.Text &= "First query: " & result & vbCrLf


The following code example demonstrates that Single(Of TSource)(IEnumerable(Of TSource), Func(Of TSource, Boolean)) throws an exception when the sequence does not contain exactly one element that satisfies the condition.


      result = String.Empty

      ' Try to get the single item in the array whose length is > 15.
      Try
         result = fruits.Single(Function(fruit) _
                                    fruit.Length > 15)
      Catch ex As System.InvalidOperationException
         result = "There is not EXACTLY ONE element whose length is > 15."
      End Try

      ' Display the result.
      outputBlock.Text &= "Second query: " & result & vbCrLf

      ' This code produces the following output:
      '
      ' First query: passionfruit
      ' Second query: There is not EXACTLY ONE element whose length is > 15.



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

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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