Enumerable.Single<TSource> Method (IEnumerable<TSource>, Func<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)

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

  • 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.