Queryable.Single(Of TSource) Method (IQueryable(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 IQueryable(Of TSource) _ ) As TSource
Type Parameters
- TSource
The type of the elements of source.
Parameters
- source
- Type: System.Linq.IQueryable(Of TSource)
An IQueryable(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 IQueryable(Of TSource). When you use instance method syntax to call this method, omit the first parameter.| Exception | Condition |
|---|---|
| ArgumentNullException | source is Nothing. |
| 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.
outputBlock.Text &= "First query: " & result & vbCrLf
Try
' Try to get the only item in the second array.
Dim fruit2 As String = fruits2.AsQueryable().Single()
outputBlock.Text &= "Second query: " + fruit2 & vbCrLf
Catch
outputBlock.Text &= "Second query: The collection does not contain exactly one element." & vbCrLf
End Try
' This code produces the following output:
' First query: orange
' Second query: The collection does not contain exactly one element.