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Queryable.Single(Of TSource) Method (IQueryable(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
Assembly:  System.Core (in System.Core.dll)

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

Type Parameters


The type of the elements of source.


Type: System.Linq.IQueryable(Of TSource)
An IQueryable(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 IQueryable(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).


source is Nothing.


source has more than one element.

The Single(Of TSource)(IQueryable(Of TSource)) method generates a MethodCallExpression that represents calling Single(Of TSource)(IQueryable(Of TSource)) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute(Of TResult)(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling Single(Of TSource)(IQueryable(Of TSource)) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the only element in source.

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

        ' Create two arrays.
        Dim fruits1() As String = {"orange"}
        Dim fruits2() As String = {"orange", "apple"}

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

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

            ' Try to get the only item in the second array.
            Dim fruit2 As String = fruits2.AsQueryable().Single()
            MsgBox("Second query: " + fruit2)
            MsgBox("Second query: The collection does not contain exactly one element.")
        End Try

        ' This code produces the following output:

        ' First query: orange
        ' Second query: The collection does not contain exactly one element.

.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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