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Enumerable.Single(Of TSource) Method (IEnumerable(Of TSource))

Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence.

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

'Declaration
<ExtensionAttribute> _
Public Shared Function Single(Of TSource) ( _
	source As IEnumerable(Of TSource) _
) 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 the single element of.

Return Value

Type: TSource
The single element of the input sequence.

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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

source is Nothing.

InvalidOperationException

The input sequence contains more than one element.

-or-

The input sequence is empty.

The Single(Of TSource)(IEnumerable(Of TSource)) method throws an exception if the input sequence is empty. To instead return Nothing when the input sequence is empty, use SingleOrDefault.

The following code example demonstrates how to use Single(Of TSource)(IEnumerable(Of TSource)) to select the only element of an array.

' Create an array that contains one item. 
Dim fruits1() As String = {"orange"}

' Get the single item in the array. 
Dim result As String = fruits1.Single()

' Display the result.
MsgBox("First query: " & result)

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

        ' Create an array that contains two items. 
        Dim fruits2() As String = {"orange", "apple"}

        result = String.Empty

        ' Try to get the 'single' item in the array. 
        Try
            result = fruits2.Single()
        Catch ex As System.InvalidOperationException
            result = "The collection does not contain exactly one element." 
        End Try 

        ' Display the result.
        MsgBox("Second query: " & result)

        ' This code produces the following output: 
        
        ' First query: orange 
        ' Second query: The collection does not contain exactly one element.

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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