Enumerable.SingleOrDefault Method

Definition

Namespace:

System.Linq

Assembly:

System.Linq.dll

Returns a single, specific element of a sequence, or a default value if that element is not found.

Overloads

SingleOrDefault<TSource>(IEnumerable<TSource>)	Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.
SingleOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)	Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.
SingleOrDefault<TSource>(IEnumerable<TSource>, TSource)	Returns the only element of a sequence, or a specified default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.
SingleOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>, TSource)	Returns the only element of a sequence that satisfies a specified condition, or a specified default value if no such element exists; this method throws an exception if more than one element satisfies the condition.

SingleOrDefault<TSource>(IEnumerable<TSource>)

Source:

Single.cs

Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource SingleOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source);

public static TSource? SingleOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);

static member SingleOrDefault : seq<'Source> -> 'Source

<Extension()>
Public Function SingleOrDefault(Of TSource) (source As IEnumerable(Of TSource)) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IEnumerable<TSource>

An IEnumerable<T> to return the single element of.

Returns

TSource

The single element of the input sequence, or default(TSource) if the sequence contains no elements.

Exceptions

ArgumentNullException

source is null.

InvalidOperationException

The input sequence contains more than one element.

Examples

The following code example demonstrates how to use SingleOrDefault<TSource>(IEnumerable<TSource>) to select the only element of an array.

string[] fruits1 = { "orange" };

string fruit1 = fruits1.SingleOrDefault();

Console.WriteLine(fruit1);

/*
 This code produces the following output:

 orange
*/

' Create an array that contains one item.
Dim fruits1() As String = {"orange"}

' Get the single item in the array or else a default value.
Dim result As String = fruits1.SingleOrDefault()

' Display the result.
Console.WriteLine($"First array: {result}")

The following code example demonstrates that SingleOrDefault<TSource>(IEnumerable<TSource>) returns a default value when the sequence is empty.

string[] fruits2 = { };

string fruit2 = fruits2.SingleOrDefault();

Console.WriteLine(
    String.IsNullOrEmpty(fruit2) ? "No such string!" : fruit2);

/*
 This code produces the following output:

 No such string!
*/

' Create an empty array.
Dim fruits2() As String = {}

result = String.Empty

' Get the single item in the array or else a default value.
result = fruits2.SingleOrDefault()

' Display the result.
Dim output As String =
IIf(String.IsNullOrEmpty(result), "No single item found", result)
Console.WriteLine($"Second array: {output}")

' This code produces the following output:
'
' First array: orange
' Second array: No single item found

Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. Instead of checking the result for the unwanted default value and then changing it if necessary, you can use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. Then, call Single<TSource>(IEnumerable<TSource>) to obtain the element. The following code example uses both techniques to obtain a default value of 1 if a collection of page numbers is empty. Because the default value for an integer is 0, which is not usually a valid page number, the default value must be specified as 1 instead. The first result variable is checked for the unwanted default value after the query has finished executing. The second result variable is obtained by using DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) to specify a default value of 1.

int[] pageNumbers = { };

// Setting the default value to 1 after the query.
int pageNumber1 = pageNumbers.SingleOrDefault();
if (pageNumber1 == 0)
{
    pageNumber1 = 1;
}
Console.WriteLine("The value of the pageNumber1 variable is {0}", pageNumber1);

// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int pageNumber2 = pageNumbers.DefaultIfEmpty(1).Single();
Console.WriteLine("The value of the pageNumber2 variable is {0}", pageNumber2);

/*
 This code produces the following output:

 The value of the pageNumber1 variable is 1
 The value of the pageNumber2 variable is 1
*/

Dim pageNumbers() As Integer = {}

' Setting the default value to 1 after the query.
Dim pageNumber1 As Integer = pageNumbers.SingleOrDefault()
If pageNumber1 = 0 Then
    pageNumber1 = 1
End If
Console.WriteLine($"The value of the pageNumber1 variable is {pageNumber1}")

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim pageNumber2 As Integer = pageNumbers.DefaultIfEmpty(1).Single()
Console.WriteLine($"The value of the pageNumber2 variable is {pageNumber2}")

' This code produces the following output:

' The value of the pageNumber1 variable is 1
' The value of the pageNumber2 variable is 1

Remarks

The default value for reference and nullable types is null.

The SingleOrDefault method does not provide a way to specify a default value. If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method as described in the Example section.

SingleOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)

Source:

Single.cs

Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource SingleOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, bool> ^ predicate);

public static TSource? SingleOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate);

static member SingleOrDefault : seq<'Source> * Func<'Source, bool> -> 'Source

<Extension()>
Public Function SingleOrDefault(Of TSource) (source As IEnumerable(Of TSource), predicate As Func(Of TSource, Boolean)) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IEnumerable<TSource>

An IEnumerable<T> to return a single element from.

predicate

Func<TSource,Boolean>

A function to test an element for a condition.

Returns

TSource

The single element of the input sequence that satisfies the condition, or default(TSource) if no such element is found.

Exceptions

ArgumentNullException

source or predicate is null.

InvalidOperationException

More than one element satisfies the condition in predicate.

Examples

The following code example demonstrates how to use SingleOrDefault<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.SingleOrDefault(fruit => fruit.Length > 10);

Console.WriteLine(fruit1);

/*
 This code produces the following output:

 passionfruit
*/

' 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 > 10.
Dim fruit1 As String =
fruits.SingleOrDefault(Function(fruit) fruit.Length > 10)

' Display the result.
Console.WriteLine($"First array: {fruit1}")

The following code example demonstrates that SingleOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) returns a default value when the sequence contains no elements that satisfy the condition.

string fruit2 =
    fruits.SingleOrDefault(fruit => fruit.Length > 15);

Console.WriteLine(
    String.IsNullOrEmpty(fruit2) ? "No such string!" : fruit2);

/*
 This code produces the following output:

 No such string!
*/

' Get the single item in the array whose length is > 15.
Dim fruit2 As String =
fruits.SingleOrDefault(Function(fruit) fruit.Length > 15)

' Display the result.
Dim output As String =
IIf(String.IsNullOrEmpty(fruit2), "No single item found", fruit2)
Console.WriteLine($"Second array: {output}")

' This code produces the following output:
'
' First array: passionfruit
' Second array: No single item found

Remarks

The default value for reference and nullable types is null.

SingleOrDefault<TSource>(IEnumerable<TSource>, TSource)

Source:

Single.cs

Returns the only element of a sequence, or a specified default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource SingleOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source, TSource defaultValue);

public static TSource SingleOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, TSource defaultValue);

static member SingleOrDefault : seq<'Source> * 'Source -> 'Source

<Extension()>
Public Function SingleOrDefault(Of TSource) (source As IEnumerable(Of TSource), defaultValue As TSource) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IEnumerable<TSource>

An IEnumerable<T> to return the single element of.

defaultValue

TSource

The default value to return if the sequence is empty.

Returns

TSource

The single element of the input sequence, or defaultValue if the sequence contains no elements.

Exceptions

ArgumentNullException

source is null.

InvalidOperationException

The input sequence contains more than one element.

SingleOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>, TSource)

Source:

Single.cs

Returns the only element of a sequence that satisfies a specified condition, or a specified default value if no such element exists; this method throws an exception if more than one element satisfies the condition.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource SingleOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, bool> ^ predicate, TSource defaultValue);

public static TSource SingleOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate, TSource defaultValue);

static member SingleOrDefault : seq<'Source> * Func<'Source, bool> * 'Source -> 'Source

<Extension()>
Public Function SingleOrDefault(Of TSource) (source As IEnumerable(Of TSource), predicate As Func(Of TSource, Boolean), defaultValue As TSource) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IEnumerable<TSource>

An IEnumerable<T> to return a single element from.

predicate

Func<TSource,Boolean>

A function to test an element for a condition.

defaultValue

TSource

The default value to return if the sequence is empty.

Returns

TSource

The single element of the input sequence that satisfies the condition, or defaultValue if no such element is found.

Exceptions

ArgumentNullException

source or predicate is null.

InvalidOperationException

More than one element satisfies the condition in predicate.