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

Parameters

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.

Type Parameters

TSource

The type of the elements of source.

Exception Condition
ArgumentNullException

source is null.

InvalidOperationException

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
    ' Try to get the only item in the second array.
    Dim fruit2 As String = fruits2.AsQueryable().Single()
    MsgBox("Second query: " + fruit2)
Catch
    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.

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 7.1
Windows Phone
Available since 8.1
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