WPF Combobox Item Update - wpf

Question

I am rather new to the WPF setup and I am running into an issue where as far as I can see I have set it up correctly to have my combobox bound to a observable collection of object.
The Combobox will update when I add or delete items. If I make a change the items in the drop down will not show any differently but if I select one that was edited it will now show the new information but only when selected.
I have set up the object class to use INotifyPropertyChanged correctly I think but it does not seem to be functioning. Going to attach the code below so that you can easily see exactly what I am trying to describe.
What I am trying to do it allow a user to push a button and have the text inside a combobox update to show the new text.
Imports System.ComponentModel
Public Class Window2
Public _names As New System.Collections.ObjectModel.ObservableCollection(Of TestClass)
Public Sub BaseLoading() Handles MyBase.Loaded
Dim AddNewItem As New TestClass
AddNewItem.groupName = "Item " + (_names.Count + 1).ToString
_names.Add(AddNewItem)
cbo_Names.SetBinding(ItemsControl.ItemsSourceProperty, New Binding With {.Source = _names})
End Sub
Private Sub button_PreviewMouseDown(sender As Object, e As MouseButtonEventArgs)
Dim AddNewItem As New TestClass
AddNewItem.groupName = "Item " + (_names.Count + 1).ToString
_names.Add(AddNewItem)
_names(0).groupName = ("Value Changed")
End Sub
End Class
Public Class TestClasss
Implements INotifyPropertyChanged
Public _groupName As String = ""
Public Property groupName As String
Get
Return _groupName.ToString
End Get
Set(value As String)
_groupName = value
onPropertyChanged(New PropertyChangedEventArgs(_groupName))
End Set
End Property
Public Event PropertyChagned(ByVal sender As Object, ByVal e As PropertyChangedEventArgs) Implements INotifyPropertyChanged.PropertyChanged
Public Sub onPropertyChanged(ByVal e As PropertyChangedEventArgs)
RaiseEvent PropertyChagned(Me, e)
End Sub
End Class
XAML
<Window x:Class="Window2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<StackPanel>
<Button x:Name="button" Content="Button" PreviewMouseDown="button_PreviewMouseDown"/>
<ComboBox x:Name="cbo_Names" Margin="30,5,30,5" IsEditable="False" ItemsSource="{Binding _names, NotifyOnSourceUpdated=True,Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" DisplayMemberPath="groupName" SelectedItem="{Binding _names, NotifyOnSourceUpdated=True,Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" />
</StackPanel>
</Window>
I would appreciate any help locating what I am missing.

Answer 1

You should pass the name of the data-bound property (instead of the value of the property) to the constructor of the PropertyChangedEventArgs:
onPropertyChanged(New PropertyChangedEventArgs("groupName"))

Answer 2

If you are using at least Visual Studio 2015, you could consider making the following change to your onPropertyChanged routine:
Public Sub onPropertyChanged(<System.Runtime.CompilerServices.CallerMemberName> Optional ByVal propertyName As String)
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
End Sub
Then, in the setter for groupName you can call onPropertyChanged without specifying the property name, and it will be taken from the name of the caller (that is, it will end up being "groupName").
Effectively, this is doing the same thing as the previous answer, but in a way that is easier for you to code and maintain. (Along with the <CallerMemberName> attribute, this works well with NameOf, both making your code more robust against any changes in names of properties.)