Добавление элементов в поле со списком в WPF на холсте

Добрый день,

Я хочу добавить определенные элементы в свой список со списком, но у меня возникает ошибка.

Код:

Add-Type -AssemblyName PresentationFramework

[xml]$Form = @"

<Window xmlns = "http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    Title = "Printer GUI" Height = "465" Width = "375">
    <Canvas Background = "White">
        <Label Name = "label_username" Content = "Username" HorizontalAlignment = "Left" Margin = "30,30,0,0" VerticalAlignment = "Top"/>
        <Label Name = "label_Location" Content = "Select Location" HorizontalAlignment = "Left" Margin = "170,30,0,0" VerticalAlignment = "Top"/>
        <TextBox Name = "textbox_username" HorizontalAlignment = "Left" Height = "23" Margin = "30,56,0,0" TextWrapping = "Wrap" VerticalAlignment = "Top" Width = "120"/>
        <ComboBox Name = "ComboBox_Location" HorizontalAlignment = "Left" Margin = "170,56,0,0" VerticalAlignment = "Top" Width = "160"/>
            <ComboBoxItem Content = "X"/>
            <ComboBoxItem Content = "Y"/>
            <ComboBoxItem Content = "Z"/>
        </ComboBox>  
        <Label Name = "label_printer" Content = "Please select printer from list" HorizontalAlignment = "Left" Margin = "30,105,0,0" VerticalAlignment = "Top"/>
        <ComboBox Name = "ComboBox_Printer" HorizontalAlignment = "Left" Margin = "30,131,0,0" VerticalAlignment = "Top" Width = "300"/>
        <CheckBox Name = "CheckBox" Content = "Check here if you want to add this printer &#xA;&#x9;to your permanent list" HorizontalAlignment = "Left" Margin = "56,185,0,0" VerticalAlignment = "Top" Height = "39" Width = "245"/>
        <Button Name = "Button_Add" Content = "Add Printer" Margin = "140,244,140,150" Width = "75" Height = "25" VerticalAlignment = "Center" ClickMode = "Release"/>
        <Button Name = "Button_XPS" Content = "Printer Defaults issue&#xD;&#xA;  in Practice Partner" Margin = "90,285,90,102" Width = "175" Height = "40" HorizontalContentAlignment = "Center" VerticalContentAlignment = "Center" ClickMode = "Release" VerticalAlignment = "Center"/>
        <Button Name = "Button_Service" Content = "Service call for Printer" Margin = "113,343,110,51" Width = "130" Height = "25" VerticalAlignment = "Center" ClickMode = "Release"/>
        <Button Name = "Button_Cancel" Content = "Cancel" Margin = "140,384,140,10" Width = "75" Height = "25" VerticalAlignment = "Center" ClickMode = "Release"/>
    </Canvas>
</Window>

"@

$NR=(New-Object System.Xml.XmlNodeReader $Form)
$Win=[Windows.Markup.XamlReader]::Load($NR)

$Win.Showdialog()

Как видите, я пытался добавлять элементы, но ошибка заключалась в следующем;

Cannot convert value "<Window xmlns=...</Window>" to type "System.Xml.XmlDocument".
Error: "The 'Canvas' start tag on line 4 position 6 does not match the end tag of 
'ComboBox'. Line 12, position 11."
At H:\Scripts\Form_Main_XAML.ps1:3 char:1
+ [xml]$Form = @"
+ ~~~~~~~~~~~~~~~
    + CategoryInfo          : MetadataError: (:) [], ArgumentTransformationMetadataException
    + FullyQualifiedErrorId : RuntimeException

Я впервые в Visual Studios, и до этого этот код был WinForms. Спасибо всем заранее.