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

May 11, 2014

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 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.

ExceptionCondition
ArgumentNullException

source or predicate is null.

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<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>) method throws an exception if the input sequence contains no matching element. To instead return null when no matching element is found, use SingleOrDefault.

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.


      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.


      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.



Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

Show:
© 2014 Microsoft