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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
)

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.

Type Parameters

TSource

The type of the elements of source.

Exception Condition
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<TSource>.

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.

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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