Binding an ObservableCollection Dependency Property in a User Control - wpf

Question

I have a UserControl that hosts a TreeView that has a custom collection which inherits from ObservableCollection of a custom class. I am trying to bind to a property using a dependency property from a ViewModel, whilst I have proven the process works for another property it does not work here.
I'm sure it is something silly I have missed but I've been going round in circles, can anyone spot my mistake?
The UserControl XAML:
<UserControl
x:Class="FileExplorer.TreeViewUserControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:fe="clr-namespace:WpfControlLibrary.FileExplorer"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
x:Name="FileExplorer"
d:DesignHeight="300"
d:DesignWidth="400"
mc:Ignorable="d">
<TreeView
BorderThickness="0"
ItemsSource="{Binding FileSystem, RelativeSource={RelativeSource AncestorType=UserControl}}"
TreeViewItem.Collapsed="TreeView_Collapsed"
TreeViewItem.Expanded="TreeView_Expanded"
VirtualizingStackPanel.IsVirtualizing="True"
VirtualizingStackPanel.VirtualizationMode="Recycling">
<TreeView.ItemTemplate>
<HierarchicalDataTemplate DataType="{x:Type fe:FileSystemObject}" ItemsSource="{Binding Items}">
<StackPanel Margin="0,2" Orientation="Horizontal">
<Image
Width="14"
Margin="2"
Source="{Binding Image}"
Stretch="Fill" />
<TextBlock
VerticalAlignment="Center"
FontSize="{StaticResource FontSizeSmall}"
Text="{Binding Name}" />
</StackPanel>
</HierarchicalDataTemplate>
</TreeView.ItemTemplate>
</TreeView>
The code behind for the UserControl:
Namespace FileExplorer
Public Class TreeViewUserControl
Inherits UserControl
#Region "Constructors"
Public Sub New()
' This call is required by the designer.
InitializeComponent()
' Add any initialization after the InitializeComponent() call.
Me.DataContext = Me
End Sub
#End Region
#Region "Properties"
Public Shared ReadOnly FileSystemProperty As DependencyProperty = DependencyProperty.Register("FileSystem", GetType(FileSystemObjectCollection), GetType(TreeViewUserControl))
Public Property FileSystem As FileSystemObjectCollection
Get
Return CType(GetValue(FileSystemProperty), FileSystemObjectCollection)
End Get
Set(ByVal value As FileSystemObjectCollection)
SetValue(FileSystemProperty, value)
FileExplorer.UpdateLayout()
End Set
End Property
#End Region
Private Function GetFileSystemInfo(ByVal root As String) As FileSystemObjectCollection
Dim items As New FileSystemObjectCollection
' Parse all the directories at the path
For Each dir As String In Directory.GetDirectories(root)
items.Add(New FileSystemObject(dir, root))
Next
' Parse all the file at the path
For Each file As String In Directory.GetFiles(root)
items.Add(New FileSystemObject(file, root))
Next
Return items
End Function
Private Sub TreeView_Collapsed(sender As Object, e As RoutedEventArgs)
Dim node As TreeViewItem = CType(e.OriginalSource, TreeViewItem)
Dim fs As FileSystemObject = CType(node.DataContext, FileSystemObject)
fs.Clear()
End Sub
Private Sub TreeView_Expanded(sender As Object, e As RoutedEventArgs)
Dim node As TreeViewItem = CType(e.OriginalSource, TreeViewItem)
Dim fs As FileSystemObject = CType(node.DataContext, FileSystemObject)
If Not fs.HasChildren Then Exit Sub
fs.Items = GetFileSystemInfo(fs.FullName)
End Sub
End Class
End Namespace
The MainWindow XAML:
<Window
x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:fe="clr-namespace:WpfControlLibrary.FileExplorer;assembly=WpfControlLibrary"
xmlns:local="clr-namespace:ModStudio.Client"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:vm="clr-namespace:ModStudio.Client.ViewModels"
Title="MainWindow"
d:DesignHeight="600"
d:DesignWidth="800"
WindowStartupLocation="CenterOwner"
WindowState="Maximized"
mc:Ignorable="d">
<Window.Resources>
<vm:MainWindowViewModel x:Key="MainWindowViewModel" />
</Window.Resources>
<Grid>
<fe:TreeViewUserControl DataContext="{StaticResource MainWindowViewModel}" FileSystem="{Binding ApplicationExplorer}" />
</Grid>
</Window>
The MainWindow has its DataContext set to a new instance of the ViewModel in code behind. The MainWindowViewModel has a property called ApplicationExplorer which is an instance of FileSystemObjectCollection. As mentioned FileSystemObjectCollection inherits from ObservableCollection(Of FileSystemObject). A FileSystemObject implements INotifyPropertyChanged. If I change ApplicationExplorer property, the control remains blank.
I have intentionally omitted some code here, but can add them if necessary.

Answer 1

Don't set the DataContext in the UserControl, i.e. remove this line:
Me.DataContext = Me
When you explcitly set the DataContext like this, you break the inheritance chain which means that the binding to the ApplicationExplorer property in your window no longer works:
<fe:TreeViewUserControl DataContext="{StaticResource MainWindowViewModel}"
FileSystem="{Binding ApplicationExplorer}" />

Answer 2

I made 3 changes and got this working!
Added OnPropertyChanged to ApplicationExplorer:
Public Property ApplicationExplorer As FileSystemObjectCollection
Get
Return _applicationExplorer
End Get
Set(value As FileSystemObjectCollection)
_applicationExplorer = value
OnPropertyChanged(NameOf(ApplicationExplorer))
End Set
End Property
Updated the binding in the UserControl:
<UserControl
x:Class="FileExplorer.TreeViewUserControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:fe="clr-namespace:WpfControlLibrary.FileExplorer"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
x:Name="FileExplorer"
d:DesignHeight="300"
d:DesignWidth="400"
mc:Ignorable="d">
<TreeView
BorderThickness="0"
ItemsSource="{Binding FileSystem, RelativeSource={RelativeSource AncestorType=fe:TreeViewUserControl}}"
TreeViewItem.Collapsed="TreeView_Collapsed"
TreeViewItem.Expanded="TreeView_Expanded"
VirtualizingStackPanel.IsVirtualizing="True"
VirtualizingStackPanel.VirtualizationMode="Recycling">
<TreeView.ItemTemplate>
<HierarchicalDataTemplate DataType="{x:Type fe:FileSystemObject}" ItemsSource="{Binding Items}">
<StackPanel Margin="0,2" Orientation="Horizontal">
<Image
Width="14"
Margin="2"
Source="{Binding Image}"
Stretch="Fill" />
<TextBlock
VerticalAlignment="Center"
FontSize="12"
Text="{Binding Name}" />
</StackPanel>
</HierarchicalDataTemplate>
</TreeView.ItemTemplate>
</TreeView>
</UserControl>
Removed the Me.DataContext = Me from the UserControl code behind as you suggested