Example of drawing a duct perpendicular to the duct using the revit api

/// <summary>
/// Select the point outside the air duct and the air duct, and draw the air duct perpendicular to the air duct.
/// </summary>
[Transaction()]
[Regeneration()]
public class cmd : IExternalCommand
{
    public Result Execute(ExternalCommandData cmdData, ref string msg, ElementSet elements)
    {
        UIDocument uiDoc = ;
        UIApplication uiApp = ;
        Document doc = ;
        Selection selection = ;

        Transaction ts = new Transaction(doc, "https://");
        ();
//Get a type of air duct
        DuctType ductType = null;
        FilteredElementCollector collector = new FilteredElementCollector(doc);
        (typeof(DuctType));
        foreach (Element el in ())
        {
            ductType = el as DuctType;
            break;
        }
        //
Reference refDuct = (, "Select a duct:");
        Duct duct = (refDuct) as Duct;
XYZ xyz = ();//Select a little
        LocationCurve lCurve = as LocationCurve;
IntersectionResult interResult = (xyz);//The projection of the point on the straight line
        if (interResult != null)
        {
            (xyz, , ductType);
        }
        ();
        return ;
    }
}