public IList<Element> findElementsByCategory(Autodesk.Revit.UI.UIApplication aApp,

     Document aDoc, BuiltInCategory ACategory)

 {

     FilteredElementCollector collector = new FilteredElementCollector(aDoc).OfCategory(

         ACategory).WhereElementIsNotElementType();

     IList<Element> oList = new List<Element>();

     foreach (Element ent in collector)

         oList.Add(ent);

     return oList;

 }

 Document doc = commandData.Application.ActiveUIDocument.Document;

 Selection sel = commandData.Application.ActiveUIDocument.Selection;

 IList<Element> oList = findElementsByCategory(commandData.Application,

     doc, BuiltInCategory.OST_Walls);

 ICollection<ElementId> oSelSet = new List<ElementId>();

 foreach (Element ent in oList)

     oSelSet.Add(ent.Id);

 sel.SetElementIds(oSelSet);