Enumerable.Single<TSource> Method (IEnumerable<TSource>, Func<TSource, Boolean>)
Microsoft Silverlight will reach end of support after October 2021. Learn more.
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)
Syntax
'Declaration
<ExtensionAttribute> _
Public Shared Function Single(Of TSource) ( _
source As IEnumerable(Of TSource), _
predicate As Func(Of TSource, Boolean) _
) As TSource
public static TSource Single<TSource>(
this IEnumerable<TSource> source,
Func<TSource, bool> predicate
)
Type Parameters
- TSource
The type of the elements of source.
Parameters
- source
Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> to return a single element from.
- predicate
Type: System.Func<TSource, Boolean>
A function to test an element for a condition.
Return Value
Type: TSource
The 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<TSource>. When you use instance method syntax to call this method, omit the first parameter.
Exceptions
| Exception | Condition |
|---|---|
| ArgumentNullException | source or predicate is nulla null reference (Nothing in Visual Basic). |
| InvalidOperationException | No element satisfies the condition in predicate. -or- More than one element satisfies the condition in predicate. -or- The source sequence is empty. |
Remarks
The Single<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>) method throws an exception if the input sequence contains no matching element. To instead return nulla null reference (Nothing in Visual Basic) when no matching element is found, use SingleOrDefault.
Examples
The following code example demonstrates how to use Single<TSource>(IEnumerable<TSource>, Func<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
string[] fruits = { "apple", "banana", "mango",
"orange", "passionfruit", "grape" };
string fruit1 = fruits.Single(fruit => fruit.Length > 10);
outputBlock.Text += fruit1 + "\n";
/*
This code produces the following output:
passionfruit
*/
The following code example demonstrates that Single<TSource>(IEnumerable<TSource>, Func<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.
string fruit2 = null;
try
{
fruit2 = fruits.Single(fruit => fruit.Length > 15);
}
catch (System.InvalidOperationException)
{
outputBlock.Text += @"The collection does not contain exactly
one element whose length is greater than 15." + "\n";
}
outputBlock.Text += fruit2 + "\n";
// This code produces the following output:
//
// The collection does not contain exactly
// one element whose length is greater than 15.
Version Information
Silverlight
Supported in: 5, 4, 3
Silverlight for Windows Phone
Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0
XNA Framework
Supported in: Xbox 360, Windows Phone OS 7.0
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.