class DuctTypeModel
    {
        [NodeName("DuctTypes")]
        [NodeCategory("DynaMEP.DynaMEP.MEP.Ducts")]
        [NodeDescription("Get All Duct Type From Model Dropdown")]
        [IsDesignScriptCompatible]
        [IsVisibleInDynamoLibrary(true)]
        public class DuctTypeUi : DSDropDownBase
        {
            const string NoDuctTypes = "No types were found.";

            /// <summary>
            /// </summary>
            public DuctTypeUi() : base("DuctType")
            {
            }

            /// <summary>
            /// </summary>
            /// <param name="currentSelection"></param>
            /// <returns></returns>
            protected override SelectionState PopulateItemsCore(string currentSelection)
            {
                Items.Clear();

                Document doc = DocumentManager.Instance.CurrentDBDocument;
                List<Element> vtList = new FilteredElementCollector(doc).OfClass(typeof(DuctType)).ToList();

                if (vtList.Count == 0)
                {
                    Items.Add(new DynamoDropDownItem(NoDuctTypes, null));
                    SelectedIndex = 0;
                    return SelectionState.Done;
                }

                foreach (var element in vtList)
                {
                    var v = (DuctType) element;
                    Items.Add(new DynamoDropDownItem(v.Name, v));
                }

                Items = Items.OrderBy(x => x.Name).ToObservableCollection();
                return SelectionState.Restore;
            }

            /// <summary>
            /// </summary>
            /// <param name="inputAstNodes"></param>
            /// <returns></returns>
            public override IEnumerable<AssociativeNode> BuildOutputAst(List<AssociativeNode> inputAstNodes)
            {
                AssociativeNode node;

                if (SelectedIndex == -1)
                {
                    node = AstFactory.BuildNullNode();
                }
                else
                {
                    DuctType ductType = Items[SelectedIndex].Item as DuctType;


                    StringNode idNode = AstFactory.BuildStringNode(ductType.Name);
                    BooleanNode falseNode = AstFactory.BuildBooleanNode(true);

                    node = AstFactory.BuildFunctionCall(
                        new Func<string, bool, object>(ElementSelector.ByUniqueId),
                        new List<AssociativeNode> {idNode, falseNode});
                }

                return new[] {AstFactory.BuildAssignment(GetAstIdentifierForOutputIndex(0), node)};
            }
        }
    }