Does 'Selection' contain a definition for 'Elements' ?

[Anonymous]

These codes are from a book about Revit API(compiled by Autodesk), but VS says that 'Selection' does not contain a definition for 'Elements' and no extension method 'Elements' accepting a first argument of type 'Selection' could be found (are you missing a using directive or an assembly reference?). So what is the problem?  Please help me~

using System;
using Autodesk.Revit.DB;
using Autodesk.Revit.UI.Selection;
using Autodesk.Revit.UI;

namespace select
{
    [Autodesk.Revit.Attributes.Transaction(Autodesk.Revit.Attributes.TransactionMode.Manual)]
    public class Class1:IExternalCommand
    {
        public Autodesk.Revit.UI.Result Execute(ExternalCommandData revit,ref string message,ElementSet elements)
        {
            try
            {
                UIDocument uIDocument = revit.Application.ActiveUIDocument;
                Selection selection = uIDocument.Selection;
                ElementSet collection = selection.Elements;
                if (collection.Size == 0)
                    TaskDialog.Show("Revit", "No elements selected");
                else
                {
                    string info = "information:";
                    foreach (ElementId elem in collection)
                        info += "\n\t" + elem.GetType().ToString();
                    TaskDialog.Show("Revit", info);
                }
            }
            catch(Exception e)
            {
                message = e.Message;
                return Autodesk.Revit.UI.Result.Failed;
            }
            return Autodesk.Revit.UI.Result.Succeeded;
        }    
    }   
}

[aignatovich · Accepted solution]

Hi!

Use Selection.GetElementIds() instead, this changed few Revit versions ago

[so-chong · Accepted solution]

Here is an example
https://knowledge.autodesk.com/search-result/caas/CloudHelp/cloudhelp/2017/ENU/Revit-API/files/GUID-...

[Anonymous]

Thank you all very much!