Remap Revit element levels so levels can be deleted without deleting geometry

[Anonymous]

    public partial class ThisApplication
    {
        private void Module_Startup(object sender, EventArgs e)
        {

        }

        private void Module_Shutdown(object sender, EventArgs e)
        {

        }

        #region Revit Macros generated code
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(Module_Startup);
            this.Shutdown += new System.EventHandler(Module_Shutdown);
        }
        #endregion
        public void LevelRemap()
        {
            
            Document doc = this.ActiveUIDocument.Document;
            UIDocument uidoc = this.ActiveUIDocument;
            Level level = doc.GetElement(uidoc.Selection.PickObject(ObjectType.Element, "Select level")) as Level;
            
            Level levelBelow = new FilteredElementCollector(doc)
                .ofClass(typeof(Level))
                .Cast<Level>()
                .OrderBy(q => q.Elevation)
                .Where(q => q.Elevation <= level.Elevation)
                .FirstOrDefault();
            
            if (levelBelow == null)
            {
                TaskDialog.Show("Error", "No level below " + level.Elevation);
                return;
            }
            
            List<string> paramsToAdjust = new List<string> { "Base offset", "Sill Height", "Top offset"};
            
            List<Element> elements = new FilteredElementCollector(doc).WherePasses(new ElementLevelFilter(level.Id)).ToList();
            using (Transaction t = new Transaction(doc, "Level Remap"))
            {
                t.Start();
                foreach (Element e in elements)
                {
                    foreach (Parameter p in e.Parameters)
                    {
                        if (p.StorageType != StorageType.ElementId  || p.IsReadOnly)
                            continue;
                        if (p.AsElementId() != level.Id)
                            continue;
                        
                        double elevationDiff = level.Elevation - levelBelow.Elevation;
                        
                        p.Set(levelBelow.Id);
                        
                        foreach (string paramName in paramsToAdjust)
                        {
                            Parameter pToAdjust = e.LookupParameter(paramName);
                            if (pToAdjust != null && !pToAdjust.IsReadOnly)
                            pToAdjust.Set(pToAdjust.AsDouble() + elevationDiff);
                        }
                    }
                }
                t.commit();
            }
    
        }

Error:'Autodesk.Revit.DB.FilteredElementCollector' does not contain a definition for 'ofClass' and no extension method 'ofClass' accepting a first argument of type 'Autodesk.Revit.DB.FilteredElementCollector' could be found (are you missing a using directive or an assembly reference?) (CS1061) - C:\ProgramData\Autodesk\Revit\Macros\2018\Revit\AppHookup\levelRemap\Source\levelRemap\ThisApplication.cs:47,6

Kindly give a solution to clear the error

[BobbyC.Jones]

OfClass()