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


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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)

Public Shared Function Single(Of TSource) (
	source As IEnumerable(Of TSource),
	predicate As Func(Of TSource, Boolean)
) As TSource


Type: System.Collections.Generic.IEnumerable(Of TSource)

An IEnumerable(Of T) to return a single element from.

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.

Type Parameters


The type of the elements of source.

Exception Condition

source or predicate is null.


No element satisfies the condition in predicate.


More than one element satisfies the condition in predicate.


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 null when no matching element is found, use SingleOrDefault(Of TSource).

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.
MsgBox("First query: " & result)

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.
    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.
MsgBox("Second query: " & result)

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

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