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

.NET Framework (current version)

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

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

Syntax

<ExtensionAttribute>
Public Shared Function Single(Of TSource) (
	source As IEnumerable(Of TSource)
) As TSource

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.

Type Parameters

TSource: The type of the elements of source.

Exceptions

Exception	Condition
ArgumentNullException	source is null.
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 null when the input sequence is empty, use SingleOrDefault(Of TSource).

Examples

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.

Version Information

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1