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
Assembly: 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: TSourceThe 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.| Exception | Condition |
|---|---|
| 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. outputBlock.Text &= "First query: " & result & vbCrLf
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.
outputBlock.Text &= "Second query: " & result & vbCrLf
' This code produces the following output:
'
' First query: passionfruit
' Second query: There is not EXACTLY ONE element whose length is > 15.
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