.NET Framework Class Library
EnumerableSingleOrDefaultTSource Method (IEnumerableTSource)

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.

Namespace:   System.Linq
Assemblies:   System.Linq (in System.Linq.dll)
  System.Core (in System.Core.dll)
Syntax
<[%$TOPIC/bb342451_en-us_VS_110_1_0_0_0_0%]> _
Public Shared Function SingleOrDefault(Of TSource) ( _
	source As [%$TOPIC/bb342451_en-us_VS_110_1_0_0_0_1%](Of TSource) _
) As TSource
public static TSource SingleOrDefault<TSource>(
	this [%$TOPIC/bb342451_en-us_VS_110_1_0_1_0_0%]<TSource> source
)
[[%$TOPIC/bb342451_en-us_VS_110_1_0_2_0_0%]]
public:
generic<typename TSource>
static TSource SingleOrDefault(
	[%$TOPIC/bb342451_en-us_VS_110_1_0_2_0_1%]<TSource>^ source
)
static member SingleOrDefault : 
        source:[%$TOPIC/bb342451_en-us_VS_110_1_0_3_0_0%]<'TSource> -> 'TSource 
JScript does not support generic types or methods.

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Collections.GenericIEnumerableTSource

An IEnumerableT to return the single element of.

Return Value

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

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerableTSource. 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).
Exceptions
ExceptionCondition
ArgumentNullException

source is .

InvalidOperationException

The input sequence contains more than one element.

Remarks

The default value for reference and nullable types is .

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 DefaultIfEmptyTSource(IEnumerableTSource, TSource) method as described in the Example section.

Examples

The following code example demonstrates how to use SingleOrDefaultTSource(IEnumerableTSource) to select the only element of an array.

' 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.
MsgBox("First array: " & result)
string[] fruits1 = { "orange" };

            string fruit1 = fruits1.SingleOrDefault();

            Console.WriteLine(fruit1);

            /*
             This code produces the following output:

             orange
            */

The following code example demonstrates that SingleOrDefaultTSource(IEnumerableTSource) returns a default value when the sequence does not contain exactly one element.

        ' 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)
        MsgBox("Second array: " & output)

        ' This code produces the following output: 
        ' 
        ' First array: orange 
        ' Second array: No single item found
string[] fruits2 = { };

            string fruit2 = fruits2.SingleOrDefault();

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

            /*
             This code produces the following output:

             No such string!
            */

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 DefaultIfEmptyTSource(IEnumerableTSource, TSource) method to specify the default value that you want to use if the collection is empty. Then, call SingleTSource(IEnumerableTSource) 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 DefaultIfEmptyTSource(IEnumerableTSource, TSource) to specify a default value of 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
        MsgBox(String.Format("The value of the pageNumber1 variable is {0}", pageNumber1))

        ' Setting the default value to 1 by using DefaultIfEmpty() in the query. 
        Dim pageNumber2 As Integer = pageNumbers.DefaultIfEmpty(1).Single()
        MsgBox(String.Format("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
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
            */
Version Information

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8