由于Revit2015后改变了 selection.Elements 的方法获取元素，因此只能通过Id来获取元素

Revit2018给出的代码如下：

[Autodesk.Revit.Attributes.Transaction(Autodesk.Revit.Attributes.TransactionMode.ReadOnly)]public class Document_Selection : IExternalCommand{    public Autodesk.Revit.UI.Result Execute(ExternalCommandData commandData,        ref string message, ElementSet elements)    {        try        {            // Select some elements in Revit before invoking this command            // Get the handle of current document.            UIDocument uidoc = commandData.Application.ActiveUIDocument;            // Get the element selection of current document.            Selection selection = uidoc.Selection;            ICollection
     
       selectedIds = uidoc.Selection.GetElementIds();            if (0 == selectedIds.Count)            {                // If no elements selected.                TaskDialog.Show("Revit","You haven't selected any elements.");            }            else            {                String info = "Ids of selected elements in the document are: ";                foreach (ElementId id in selectedIds)                {                   info += "\n\t" + id.IntegerValue;                }                TaskDialog.Show("Revit",info);            }        }        catch (Exception e)        {            message = e.Message;            return Autodesk.Revit.UI.Result.Failed;        }        return Autodesk.Revit.UI.Result.Succeeded;    }    /// }