Information
The topic you requested is included in another documentation set. For convenience, it's displayed below. Choose Switch to see the topic in its original location.

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)

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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

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);

Console.WriteLine(fruit1);

/*
 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)
{
    Console.WriteLine(@"The collection does not contain exactly 
                    one element whose length is greater than 15.");
}

Console.WriteLine(fruit2);

// This code produces the following output: 
// 
// The collection does not contain exactly   
// one element whose length is greater than 15.

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0

Community Additions

Show:
© 2014 Microsoft