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

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


Type: System.Linq.IQueryable(Of TSource)

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

Type: System.Linq.Expressions.Expression(Of 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 the condition in predicate.

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.

This method has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).

The Single(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) method generates a MethodCallExpression that represents calling Single(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) 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), Expression(Of Func(Of TSource, Boolean))) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the only element in source that satisfies the condition specified by predicate.

The following code example demonstrates how to use Single(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) to select the only element of an array that satisfies a condition.

Dim fruits() As String = _
    {"apple", "banana", "mango", "orange", "passionfruit", "grape"}

' Get the only string in the array whose length is greater than 10.
Dim result As String = _
    fruits.AsQueryable().Single(Function(fruit) fruit.Length > 10)

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

    ' Try to get the only string in the array
    ' whose length is greater than 15.
    Dim fruit2 As String = fruits.AsQueryable().Single(Function(fruit) fruit.Length > 15)
    MsgBox("Second Query: " + fruit2)
    Dim text As String = "Second Query: The collection does not contain "
    text = text & "exactly one element whose length is greater than 15."
End Try

' This code produces the following output:

' First Query: passionfruit
' Second Query: The collection does not contain exactly one 
'   element whose length is greater than 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 7.1
Windows Phone
Available since 8.1
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