Queryable.SingleOrDefault(Of TSource) Method (IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean)))

 
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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)

<ExtensionAttribute>
Public Shared Function SingleOrDefault(Of TSource) (
	source As IQueryable(Of TSource),
	predicate As Expression(Of Func(Of TSource, Boolean))
) As TSource

Parameters

source
Type: System.Linq.IQueryable(Of TSource)

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

predicate
Type: System.Linq.Expressions.Expression(Of 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 in predicate, or default(TSource) if no such element is found.

Type Parameters

TSource

The type of the elements of source.

Exception Condition
ArgumentNullException

source or predicate is null.

InvalidOperationException

More than one element satisfies the condition in predicate.

This method has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).

The SingleOrDefault(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) method generates a MethodCallExpression that represents calling SingleOrDefault(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute(Of TResult)(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling SingleOrDefault(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the only element in source that satisfies the condition specified by predicate, or a default value if no such element exists.

The following code example demonstrates how to use SingleOrDefault(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) to select the only element of an array that satisfies a condition. The second query demonstrates that SingleOrDefault(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) returns a default value when the sequence does not contain exactly one element that satisfies the condition.

Dim fruits() As String = _
    {"apple", "banana", "mango", "orange", "passionfruit", "grape"}

' Get the single string in the array whose length is greater
' than 10, or else the default value for type string (null).
Dim fruit1 As String = _
    fruits.AsQueryable().SingleOrDefault(Function(fruit) fruit.Length > 10)
' Display the result.
MsgBox("First Query: " & fruit1)

' Get the single string in the array whose length is greater
' than 15, or else the default value for type string (null).
Dim fruit2 As String = _
    fruits.AsQueryable().SingleOrDefault(Function(fruit) fruit.Length > 15)
MsgBox("Second Query: " & _
    IIf(String.IsNullOrEmpty(fruit2), "No such string!", fruit2))

' This code produces the following output:

' First Query: passionfruit
' Second Query: No such string!

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 7.1
Windows Phone
Available since 8.1
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