Data Grid not updating from ViewModel - wpf

Question

Do you know why looking at the code in the XAML and ViewModel why a Data Grid wont update when the an item is added? I know the DataGrid binding is correct as will add an item if I add it by code, however I am adding a item via a view.
Also the getter of the ObservableCollection is getting hit every time I do add a item.
Thanks
------ Code ------
XAML
<Window x:Class="Importer.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Product Group" Height="350" Width="721"
xmlns:VM="clr-namespace:Importer.ViewModels">
<Window.DataContext>
<VM:ProductsViewModel />
</Window.DataContext>
<Grid Margin="0,0,0.4,0.4">
<DataGrid x:Name="dgGrid" ColumnWidth="Auto" Margin="10,10,0,0" VerticalAlignment="Top"
ItemsSource="{Binding ProductCollection}" HorizontalAlignment="Left" AutoGenerateColumns="False" >
<DataGrid.Columns>
<DataGridTextColumn Binding="{Binding Name}" Header="Name" Width="150"/>
<DataGridTextColumn Binding="{Binding Description}" Header="Description" Width="400" />
</DataGrid.Columns>
</DataGrid>
<Button x:Name="btnAddProjectGroup" Content="Add Project Group" HorizontalAlignment="Left" Margin="10,277,0,0" VerticalAlignment="Top" Width="135" Click="btnAddProjectGroup_Click"/>
</Grid>
</Window>
VIEW 1 (that uses the XAML above)
public partial class MainWindow : Window
{
ProductsViewModel m_VM = new ProductsViewModel();
public MainWindow()
{
InitializeComponent();
}
private void btnAddProjectGroup_Click(object sender, RoutedEventArgs e)
{
// Open new window here to add a new project group
AddProductGroup dlg = new AddProductGroup(m_VM);
dlg.ShowDialog();
}
}
VIEW 2 - The view that passes the values to add to the collection
public partial class AddProductGroup : Window
{
ProductGroupBindable newProduct = new ProductGroupBindable();
ProductsViewModel viewModel = null;
public AddProductGroup(ProductsViewModel vm)
{
viewModel = vm;
InitializeComponent();
}
private void btnOK_Click(object sender, RoutedEventArgs e)
{
newProduct.Id = System.Guid.NewGuid();
newProduct.Name = txtName.Text;
newProduct.Description = txtDescription.Text;
viewModel.AddProduct(newProduct);
this.Close();
}
}
VIEWMODEL
private ObservableCollection<ProductGroupBindable> m_ProductCollection;
public ProductsViewModel()
{
if (m_ProductCollection == null)
m_ProductCollection = new ObservableCollection<ProductGroupBindable>();
}
public ObservableCollection<ProductGroupBindable> ProductCollection
{
get
{
return m_ProductCollection;
}
set
{
m_ProductCollection = value;
}
}
private ObservableCollection<ProductGroupBindable> Test()
{
m_ProductCollection.Add(new ProductGroupBindable { Description = "etc", Name = "test12" });
m_ProductCollection.Add(new ProductGroupBindable { Description = "etc", Name = "test123" });
return ProductCollection;
}
public void AddProduct(ProductGroupBindable newProduct)
{
m_ProductCollection.Add(newProduct);
//NotifyPropertyChanged("ProductCollection");
}

Answer 1

You set DataContext in XAML:
<Window.DataContext>
<VM:ProductsViewModel />
</Window.DataContext>
and your dialog works on another instance of ProductsViewModel which you create in code:
ProductsViewModel m_VM = new ProductsViewModel();
Easiest workaround would be to pass DataContext to your AddProductGroup dialog:
AddProductGroup dlg = new AddProductGroup(this.DataContext as ProductsViewModel);
EDIT
or instead of setting DataContext in XAML do set it in code:
public MainWindow()
{
InitializeComponent();
this.DataContext = m_VM;
}