.net – Binding ItemsSource of a ComboBoxColumn in WPF DataGrid

I have two simple Model classes and a ViewModel…

public class GridItem
{
    public string Name { get; set; }
    public int CompanyID { get; set; }
}

public class CompanyItem
{
    public int ID { get; set; }
    public string Name { get; set; }
}

public class ViewModel
{
    public ViewModel()
    {
        GridItems = new ObservableCollection<GridItem>() {
            new GridItem() { Name = "Jim", CompanyID = 1 } };

        CompanyItems = new ObservableCollection<CompanyItem>() {
            new CompanyItem() { ID = 1, Name = "Company 1" },
            new CompanyItem() { ID = 2, Name = "Company 2" } };
    }

    public ObservableCollection<GridItem> GridItems { get; set; }
    public ObservableCollection<CompanyItem> CompanyItems { get; set; }
}

…and a simple Window:

<Window x:Class="DataGridComboBoxColumnApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <DataGrid AutoGenerateColumns="False" ItemsSource="{Binding GridItems}" >
            <DataGrid.Columns>
                <DataGridTextColumn Binding="{Binding Name}" />
                <DataGridComboBoxColumn ItemsSource="{Binding CompanyItems}"
                                    DisplayMemberPath="Name"
                                    SelectedValuePath="ID"
                                    SelectedValueBinding="{Binding CompanyID}" />
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>

The ViewModel is set to the MainWindow's DataContext in App.xaml.cs:

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        MainWindow window = new MainWindow();
        ViewModel viewModel = new ViewModel();

        window.DataContext = viewModel;
        window.Show();
    }
}

As you can see I set the ItemsSource of the DataGrid to the GridItems collection of the ViewModel. This part works, the single Grid line with Name "Jim" is displayed.

I also want to set the ItemsSource of the ComboBox in every row to the CompanyItems collection of the ViewModel. This part does not work: The ComboBox remains empty and in the Debugger Output Window I see an error message:

System.Windows.Data Error: 2 : Cannot
find governing FrameworkElement or
FrameworkContentElement for target
element.
BindingExpression:Path=CompanyItems;
DataItem=null; target element is
'DataGridComboBoxColumn'
(HashCode=28633162); target property
is 'ItemsSource' (type 'IEnumerable')

I believe that WPF expects CompanyItems to be a property of GridItem which is not the case, and that's the reason why the binding fails.

I've already tried to work with a RelativeSource and AncestorType like so:

<DataGridComboBoxColumn ItemsSource="{Binding CompanyItems, 
    RelativeSource={RelativeSource Mode=FindAncestor,
                                   AncestorType={x:Type Window}}}"
                        DisplayMemberPath="Name"
                        SelectedValuePath="ID"
                        SelectedValueBinding="{Binding CompanyID}" />

But that gives me another error in the debugger output:

System.Windows.Data Error: 4 : Cannot
find source for binding with reference
'RelativeSource FindAncestor,
AncestorType='System.Windows.Window',
AncestorLevel='1''.
BindingExpression:Path=CompanyItems;
DataItem=null; target element is
'DataGridComboBoxColumn'
(HashCode=1150788); target property is
'ItemsSource' (type 'IEnumerable')

Question: How can I bind the ItemsSource of the DataGridComboBoxColumn to the CompanyItems collection of the ViewModel? Is it possible at all?

Thank you for help in advance!