Selecting First element with IQueryable

[Lenn86]

Hi Guys,
I am working on a Windows Phone Contacts app and I am trying to make a query of the first IEnumerable element in the variable "contacts".
The output has to be shown in the Listbox under the variable "firstperson".
Currently the output returns all elements even when I added .First() after the firstperson variabele.

Hope you guys can help me out!

Here is my code:

Imports System
Imports System.Threading
Imports System.Windows.Controls
Imports Microsoft.Phone.Controls
Imports Microsoft.Phone.Shell
Imports System.IO
Imports System.Xml.Serialization
Imports System.Linq
Imports System.Xml.Linq

Partial Public Class MainPage
    Inherits PhoneApplicationPage

    ' Constructor
    Public Sub New()
        InitializeComponent()

        SupportedOrientations = SupportedPageOrientation.Portrait Or SupportedPageOrientation.Landscape

        'Uitlezen en sorteren elementen op "Voornaam" Contacten.xml
        Dim document = XDocument.Load("Assets\Contacten.xml")
        Dim element As XElement = XElement.Load("Assets\Contacten.xml", LoadOptions.None)
        Dim elementSorted As XElement() = element.Elements("Persoon").OrderBy(Function(t) CStr(t.Element("Voornaam"))).ToArray()

        Dim contacts = elementSorted.[Select](Function(t) New Contact() With {
            .Achternaam = t.Element("Achternaam").Value,
            .Voornaam = t.Element("Voornaam").Value,
            .Emailadres = t.Element("Emailadres").Value
        })

        Dim firstperson As IQueryable(Of Contact) = contacts.AsQueryable().Where(Function(person) person.Voornaam.Any())
        firstperson.First()

        ContactListBox.DataContext = firstperson

    End Sub


    Public Class Contact

        <XmlElement("Achternaam")>
        Public Property Achternaam As String

        <XmlElement("Voornaam")>
        Public Property Voornaam() As String

        <XmlElement("Emailadres")>
        Public Property Emailadres() As String

        <XmlElement("Geboortedatum")>
        Public Property Geboortedatum() As String

        <XmlElement("Telefoon")>
        Public Property Telefoon() As String

        <XmlElement("Fotobestand")>
        Public Property Fotobestand() As String

        <XmlElement("Fotolink")>
        Public Property Fotolink() As String



    End Class

End Class

[jmcilhinney]

You get the first item in the list but you don't put it anywhere, so it is immediately discarded. You then proceed to use the whole list. You should start by using variable names that actually describe what they refer to. Your firstperson variable doesn't actually refer to the first person at all. If you were to get rid of the Where call and just use First, which does the filtering and selecting, then it would be a reasonable name:

Dim firstPerson = contacts.AsQueryable().First(Function(person) person.Voornaam.Any())
 
ContactListBox.DataContext = firstPerson

[Lenn86]

Hi jmcilhinney,
Thanks for your fast reply!
The weird thing is, when I try your suggestion the listbox doesn't give any output, while it does with the "Where"-construction (this shows all items):

Attachment 179576

For what its worth I placed the XAML code of the Listbox (output) where the binding elements are:

<Run Text="{Binding Voornaam}"/>
<Run Text="{Binding Achternaam}"/>

<Grid x:Name="SavedContactsPanel" Margin="0,306,0,0">
            <TextBlock Text="Opgeslagen contacten:" HorizontalAlignment="Left" TextWrapping="Wrap" Style="{StaticResource PhoneTextNormalStyle}" FontSize="25"  Width="405" Height="41" VerticalAlignment="Top" Margin="41,10,0,0" />
            <ListBox x:Name="ContactListBox" ItemsSource="{Binding}" MaxHeight="{Binding ElementName=ContactListBox, Path=ActualHeight}" HorizontalAlignment="Left" VerticalAlignment="Stretch" Margin="28,51,0,10" Width="405" >
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <ScrollViewer VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto">
                    <StackPanel Grid.Row="0" Orientation="Horizontal" >
                        <TextBlock  Style="{StaticResource PhoneTextSubtleStyle}"  FontSize="25" >
                    		<Run Text="{Binding Voornaam}"/>
                            <Run Text="{Binding Achternaam}"/>
                        </TextBlock>
                    </StackPanel>
                           </ScrollViewer>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
        </Grid>

If there are any other ways to obtain the first element from my xml file, they are welcome

[dday9]

Welcome to VBForums, Lenn86!

I wanted to give you a heads up that your first couple of posts will need to be approved by a moderator so they won't immediately show up. However, once you post once, there's no need to post again.

Also, be sure to make use of code tags. You'll noticed that both of your posts have edits to them, that was me adding code tags. If you need help on using them, visit the link in my signature.

Edit - Also, I moved this thread to the WPF forum.

[Sherin]

Hello, @Lenn86
Please try this code, To Selecting First element with IQueryable

public static TSource First<TSource> (this System.Linq.IQueryable<TSource> source);

I hope this code will be useful to you.
Thank you.

[jmcilhinney]

Hello, @Lenn86
Please try this code, To Selecting First element with IQueryable

public static TSource First<TSource> (this System.Linq.IQueryable<TSource> source);

I hope this code will be useful to you.
Thank you.

The OP can't "try that code". That's the declaration of the method they need to call. It doesn't indicate at all how they should call it. Fortunately, I demonstrated how to call that method over two weeks ago, so I'm just wondering exactly what value you thought this post would provide. Perhaps you aren't aware that a moderator has removed the advertising from your signature so posting useless information isn't a actually working to sneak in spam.

[Lenn86]

Yeah the .Take(1) shows the first item after deserialization, but still weird why indexation ( e.g. .First) still does not work??
The program runs correctly (Option Explicit is on), the list is displayed in the control, but when I use indexation, the listbox control doesn't display anything.
I think the indexation somehow can't be applied after deserialization (maybe invalid corresponding XMLElements??)
The updated code below:

Class Persoon

<XmlRoot(ElementName:="Adresboek")>
    Public Class Persoon

    Private _achternaam As String
    Private _voornaam As String
    Private _emailadres As String


    <XmlElement(ElementName:="Achternaam")>
    Public Property Achternaam() As String
        Get
            Return _achternaam
        End Get
        Set(ByVal value As String)
            _achternaam = value
        End Set
    End Property

    <XmlElement(ElementName:="Voornaam")>
    Public Property Voornaam() As String
        Get
            Return _voornaam
        End Get
        Set(ByVal value As String)
            _voornaam = value
        End Set
    End Property

    <XmlElement(ElementName:="Emailadres")>
    Public Property Emailadres() As String
        Get
            Return _emailadres
        End Get
        Set(ByVal value As String)
            _emailadres = value
        End Set
    End Property


End Class

The Mainpage class:

Partial Public Class MainPage
    Inherits PhoneApplicationPage


    ' Constructor
    Public Sub New()
        InitializeComponent()

        SupportedOrientations = SupportedPageOrientation.Portrait Or SupportedPageOrientation.Landscape



        'Write default XML data to XML-file
        Dim xmlWriterSettings As XmlWriterSettings = New XmlWriterSettings()
        xmlWriterSettings.Indent = True

        Using myIsolatedStorage As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication()

            Using writeStream As IsolatedStorageFileStream = myIsolatedStorage.OpenFile("Contacten.xml", FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite)
                Dim serializer As XmlSerializer = New XmlSerializer(GetType(List(Of Persoon)))

                Using xmlWriter As XmlWriter = xmlWriter.Create(writeStream, xmlWriterSettings)
                    serializer.Serialize(xmlWriter, GeneratePersonData())
                End Using
            End Using
        End Using

        'Read and sort XML data by Voornaam to Object Persoon and show first item in list
        Try

            Using myIsolatedStorage As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication()

                Using readStream As IsolatedStorageFileStream = myIsolatedStorage.OpenFile("Contacten.xml", FileMode.Open)
                    Dim serializer As XmlSerializer = New XmlSerializer(GetType(List(Of Persoon)))

                    Dim data As List(Of Persoon) = CType(serializer.Deserialize(readStream), List(Of Persoon))
                    Dim dataSorted = data.OrderBy(Function(x) x.Voornaam).ToList
                    Dim contactList As New List(Of Persoon)
                    contactList = dataSorted
                    Dim first = contactList(0)
                    ContactListBox.DataContext = first
                End Using
            End Using

        Catch
        End Try



        'Dim path As String = "Assets\Contacten.xml"
        'Dim serializer = New XmlSerializer(GetType(Adresboek))
        'Dim streamReader = New FileStream(path, FileMode.Open)
        'Dim readContainer = TryCast(serializer.Deserialize(streamReader), Adresboek)
        'ContactListBox.DataContext = readContainer
        'streamReader.Close()

    End Sub

    'Generate default list Persoon and add items to list
    Private Function GeneratePersonData() As List(Of Persoon)
        Dim data As List(Of Persoon) = New List(Of Persoon)()
        data.Add(New Persoon() With {
            .Achternaam = "van Gaal",
            .Voornaam = "Louis",
            .Emailadres = "louisvangaal@prive.nl"
        })
        data.Add(New Persoon() With {
            .Achternaam = "Kluivert",
            .Voornaam = "Patrick",
            .Emailadres = "patrickkluivert@prive.nl"
        })
        data.Add(New Persoon() With {
            .Achternaam = "van Persie",
            .Voornaam = "Robin",
            .Emailadres = "robinvanpersie@prive.nl"
        })
        data.Add(New Persoon() With {
            .Achternaam = "Robben",
            .Voornaam = "Arjen",
            .Emailadres = "arjenrobben@prive.nl"
        })
        Return data
    End Function

End Class

[Lenn86]

So when I use the code below all items are displayed correctly:


The Mainpage class:

Partial Public Class MainPage
    Inherits PhoneApplicationPage


    ' Constructor
    Public Sub New()
        InitializeComponent()

        SupportedOrientations = SupportedPageOrientation.Portrait Or SupportedPageOrientation.Landscape



        'Write default XML data to XML-file
        Dim xmlWriterSettings As XmlWriterSettings = New XmlWriterSettings()
        xmlWriterSettings.Indent = True

        Using myIsolatedStorage As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication()

            Using writeStream As IsolatedStorageFileStream = myIsolatedStorage.OpenFile("Contacten.xml", FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite)
                Dim serializer As XmlSerializer = New XmlSerializer(GetType(List(Of Persoon)))

                Using xmlWriter As XmlWriter = xmlWriter.Create(writeStream, xmlWriterSettings)
                    serializer.Serialize(xmlWriter, GeneratePersonData())
                End Using
            End Using
        End Using

        'Read and sort XML data by Voornaam to Object Persoon and show first item in list
        Try

            Using myIsolatedStorage As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication()

                Using readStream As IsolatedStorageFileStream = myIsolatedStorage.OpenFile("Contacten.xml", FileMode.Open)
                    Dim serializer As XmlSerializer = New XmlSerializer(GetType(List(Of Persoon)))

                    Dim data As List(Of Persoon) = CType(serializer.Deserialize(readStream), List(Of Persoon))
                    Dim dataSorted = data.OrderBy(Function(x) x.Voornaam).ToList
                    ContactListBox.DataContext = dataSorted
                    'ContactListBox.DataContext = dataSorted.Take(1) also works
                End Using
            End Using

        Catch
        End Try



        
    End Sub

    'Generate default list Persoon and add items to list
    Private Function GeneratePersonData() As List(Of Persoon)
        Dim data As List(Of Persoon) = New List(Of Persoon)()
        data.Add(New Persoon() With {
            .Achternaam = "van Gaal",
            .Voornaam = "Louis",
            .Emailadres = "louisvangaal@prive.nl"
        })
        data.Add(New Persoon() With {
            .Achternaam = "Kluivert",
            .Voornaam = "Patrick",
            .Emailadres = "patrickkluivert@prive.nl"
        })
        data.Add(New Persoon() With {
            .Achternaam = "van Persie",
            .Voornaam = "Robin",
            .Emailadres = "robinvanpersie@prive.nl"
        })
        data.Add(New Persoon() With {
            .Achternaam = "Robben",
            .Voornaam = "Arjen",
            .Emailadres = "arjenrobben@prive.nl"
        })
        Return data
    End Function

End Class

[Lenn86]

I found out that deserialization isn't the problem.
When I declare a local list and want to display in the control have the same indexation issue...
The program runs correctly without applying index to the list, all items are shown:


But when I place an index after the list like "ContactListBox.DataContext = list(0)"
or ContactListBox.DataContext = list.First()
it doesn't show any results:

[jmcilhinney]

For a start, the First method doesn't involve any indexing. First involves enumerating. Basically, a For Each loop is used to enumerate the list and the current item is returned in the first iteration.

As for the issue, the problem is presumably that the DataContext property requires that you assign a list to it, not a single item. If you only want to display a single item then create a list that contains it and assign that.

[Lenn86]

"I was blind, now I can see" This problem is solved, thanks for all your assistance jmcihinney!
Greetings and best wishes from the Netherlands.