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Enumerable.SingleOrDefault(Of TSource) Method (IEnumerable(Of TSource), Func(Of 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.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)
'Declaration
<ExtensionAttribute> _
Public Shared Function SingleOrDefault(Of TSource) ( _
	source As IEnumerable(Of TSource), _
	predicate As Func(Of TSource, Boolean) _
) As TSource
'Usage
Dim source As IEnumerable(Of TSource)
Dim predicate As Func(Of TSource, Boolean)
Dim returnValue As TSource

returnValue = source.SingleOrDefault(predicate)

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable(Of TSource)

An IEnumerable(Of T) to return a single element from.

predicate
Type: System.Func(Of TSource, Boolean)

A function to test an element for a condition.

Return Value

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

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable(Of 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 Nothing.

InvalidOperationException

More than one element satisfies the condition in predicate.

The default value for reference and nullable types is Nothing.

The following code example demonstrates how to use SingleOrDefault(Of TSource)(IEnumerable(Of TSource), Func(Of TSource, Boolean)) to select the only element of an array that satisfies a condition.

' 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.
MsgBox("First array: " & fruit1)

The following code example demonstrates that SingleOrDefault(Of TSource)(IEnumerable(Of TSource), Func(Of TSource, Boolean)) returns a default value when the sequence does not contain exactly one element that satisfies the condition.

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

' This code produces the following output: 

' First array: passionfruit 
' Second array: No single item found

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
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