Enumerable.Single(Of TSource) Method (IEnumerable(Of TSource))
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence.
Assembly: 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: TSourceThe 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.| Exception | Condition |
|---|---|
| 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.
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.
outputBlock.Text &= "Second query: " & result & vbCrLf
' This code produces the following output:
'
' First query: orange
' Second query: The collection does not contain exactly one element.