private void Initialize() { treeView.ItemsSource = StoreCollection.GetData(); var ce = new CustomEngine(); var cl = new CustomLexer(); editor.SetCustomEngine(ce, cl); editor.DataSource = treeView.ItemsSource; editor.Expression = "[Name] + \": \" + [ProductsGroups].Where(x => x.Name == \"Notebooks\").Sum(x => x.Products.Sum(p => p.Count))"; }