How to determine if an item is the last one in a WPF ItemTemplate?

I have some XAML:

<ItemsControl Name="mItemsControl">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <TextBox Text="{Binding Mode=OneWay}" KeyUp="TextBox_KeyUp"/>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

that's bound to a simple ObservableCollection

private ObservableCollection<string> mCollection = new ObservableCollection<string>();

public MainWindow()
{
    InitializeComponent();

    this.mCollection.Add("Test1");
    this.mCollection.Add("Test2");
    this.mItemsControl.ItemsSource = this.mCollection;
}

Upon hitting the enter key in the last TextBox, I want another TextBox to appear. I have code that does it, but there's a gap:

private void TextBox_KeyUp(object sender, KeyEventArgs e)
{
    if (e.Key != Key.Enter)
    {
        return;
    }

    TextBox textbox = (TextBox)sender;

    if (IsTextBoxTheLastOneInTheTemplate(textbox))
    {
        this.mCollection.Add("A new textbox appears!");
    }
}

The function IsTextBoxTheLastOneInTheTemplate() is something that I need, but can't figure out how to write. How would I go about writing it?

I've considered using ItemsControl.ItemContainerGenerator, but can't put all the pieces together.

Solution

I was able to get a decent solution by referring to http://drwpf.com/blog/2008/07/20/itemscontrol-g-is-for-generator/. Not super-elegant, but it worked for me.

    private void TextBox_KeyUp(object sender, KeyEventArgs e)
    {
        if (e.Key != Key.Enter)
        {
            return;
        }

        TextBox textbox = (TextBox)sender;

        var lastContainer = this.mItemsControl.ItemContainerGenerator.ContainerFromIndex(this.mItemsControl.Items.Count - 1);

        var visualContainer = (Visual)lastContainer;

        var containedTextbox = (TextBox)GetDescendantByType(visualContainer, typeof(TextBox));

        var isSame = textbox == containedTextbox;

        if (isSame)
        {
             this.mCollection.Add("A new textbox appears!");
        }
    }


    public static Visual GetDescendantByType(Visual element, Type type)
    {
        if (element.GetType() == type) return element;

        Visual foundElement = null;

        if (element is FrameworkElement)
            (element as FrameworkElement).ApplyTemplate();

        for (int i = 0;
            i < VisualTreeHelper.GetChildrenCount(element); i++)
        {
            Visual visual = VisualTreeHelper.GetChild(element, i) as Visual;
            foundElement = GetDescendantByType(visual, type);
            if (foundElement != null)
                break;
        }

        return foundElement;
    }