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

Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

'Declaration
<ExtensionAttribute> _
Public Shared Function FirstOrDefault(Of TSource) ( _
	source As IQueryable(Of TSource), _
	predicate As Expression(Of Func(Of TSource, Boolean)) _
) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable(Of TSource)
An IQueryable(Of T) to return an element from.
predicate
Type: System.Linq.Expressions.Expression(Of Func(Of TSource, Boolean))
A function to test each element for a condition.

Return Value

Type: TSource
default(TSource) if source is empty or if no element passes the test specified by predicate; otherwise, the first element in source that passes the test specified by predicate.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable(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.

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 FirstOrDefault(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) method generates a MethodCallExpression that represents calling FirstOrDefault(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 FirstOrDefault(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 first element in source that satisfies the condition in predicate, or a default value if no element satisfies the condition.

The following code example demonstrates how to use FirstOrDefault(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) by passing in a predicate. In the second query, there is no element in the sequence that satisfies the condition.


        Dim names() As String = {"Hartono, Tommy", "Adams, Terry", _
                             "Andersen, Henriette Thaulow", _
                             "Hedlund, Magnus", "Ito, Shu"}

        ' Get the first string in the array that is longer
        ' than 20 characters, or the default value for type
        ' string (null) if none exists.
        Dim firstLongName As String = _
                    names.AsQueryable().FirstOrDefault(Function(name) name.Length > 20)

        MsgBox(String.Format("The first long name is '{0}'.", firstLongName))

        ' Get the first string in the array that is longer
        ' than 30 characters, or the default value for type
        ' string (null) if none exists.
        Dim firstVeryLongName As String = _
            names.AsQueryable().FirstOrDefault(Function(name) name.Length > 30)

        MsgBox(String.Format( _
            "There is {0} name that is longer than 30 characters.", _
            IIf(String.IsNullOrEmpty(firstVeryLongName), "NOT a", "a")))

        ' This code produces the following output:
        '
        ' The first long name is 'Andersen, Henriette Thaulow'.
        ' There is NOT a name that is longer than 30 characters.



.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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