WPF Validating unbound textbox

The solution I use is to wire-up the controls to update their own binding sources whenever they lose focus. This makes the controls validate after the user has tabbed or clicked through them once.

public AddCustomerWindow()
{
    InitializeComponent();
    this.Loaded += AddCustomerWindow_Loaded;
    gvAddCustomer.DataContext = new Customer();  
}

void AddCustomerWindow_Loaded(object sender, RoutedEventArgs e)
{
    // Manually wire up textboxes of specific interest.
    UpdateSourceOnLostFocus(new TextBox[] {txtFirstName, txtLastName},
                            TextBox.TextProperty);

    // Or, wire up ALL controls of a given type with the help of Telerik's
    // ChildrenOfType extension, using Linq to filter the list where appropriate.
    UpdateSourceOnLostFocus(this.ChildrenOfType<RadDatePicker>()
                                .Where(picker => !picker.IsReadOnly), 
                            RadDatePicker.SelectedValueProperty);
}

void UpdateSourceOnLostFocus<T>(IEnumerable<T> controls, DependencyProperty prop) where T : Control
{
    controls.ToList()
        .ForEach(ctrl =>
        {
            var binding = BindingOperations.GetBindingExpression(ctrl, prop);
            if (binding != null)
                ctrl.LostFocus += (sender, args) => binding.UpdateSource();
        });
}

If you also want the controls to validate on load - i.e., before the user tabs/clicks through them - then you could write an UpdateSourceNow method similar to the one above, and call it from AddCustomerWindow_Loaded instead.

However, I prefer to wait until each control is visited, because it can be somewhat alarming to users if they immediately see a screen full of red boxes when they want to enter a new Model object instance.

If it is pure XML then you can use XSL and XSLT software to do the transformation.

The XSL transformation would have to parse the Text string value into the desired XML fields. It is possible, but neither simple nor straightforward. It seems strange to mix non-XML structure (the Text attribute value) with XML. I can see why you want to do the transformation (if only to leverage the vast numbers of software tools for handling XML data).

Another possibility is to write a small script (in Python or Java) that reads the XML content for you, then performs the conversion. This will probably be the most flexible solution.

An interesting read about XAML conversion: http://simonwillison.net/2003/Oct/24/microsoftsXUL/