public IActionResult TreeGridXML()
{
    return View();
}

public ActionResult GetProductsByCategory([C1JsonRequest] CollectionViewRequest<TCategory> requestData)
{
    var items = new List<TCategory>();
    var xml = XElement.Load(Path.Combine(Directory.GetCurrentDirectory(),@"wwwroot\data\ProductCategories.xml"));
    // get categories
    var categories = xml.Elements("category");
    foreach (var cg in categories)
    {
        items.Add(new TCategory
        {
            Id = Convert.ToInt32(cg.Attribute("id").Value),
            Name = cg.Attribute("name").Value,
            Products = new List<TProduct>()
        });
        // get products in this category
        var products = cg.Elements("product");
        foreach (var p in products)
        {
            items[items.Count - 1].Products.Add(new TProduct
            {
                Id = Convert.ToInt32(p.Attribute("id").Value),
                Name = p.Attribute("name").Value,
                Price = Convert.ToDouble(p.Attribute("price").Value)
            });
        }
    }
    return this.C1Json(CollectionViewHelper.Read(requestData, items));
}

public class TCategory
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<TProduct> Products { get; set; }
}

public class TProduct
{
    public int Id { get; set; }
    public string Name { get; set; }
    public double Price { get; set; }
}