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

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.
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. 
        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.
        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.

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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