//获取选中对象

[TransactionAttribute(Autodesk.Revit.Attributes.TransactionMode.Manual)]
public class setSelectEle : IExternalCommand
{

    public Result Execute(ExternalCommandData commandData, ref string messages, ElementSet elements)
    {
          SetSelectionElement(commandData, elements);
           return Result.Succeeded;
    }


    public void SetSelectionElement(ExternalCommandData   commandData, ElementSet elements)
    {
        UIApplication app = commandData.Application;
        Document doc = app.ActiveUIDocument.Document;
        //获取id为729401的Element
        ElementId el = new ElementId(729401);
　　　　 Element e = doc.GetElement(el);

        Selection sel = app.ActiveUIDocument.Selection;  
　　　　 Transaction trans = new Transaction(doc); 
　　　　 trans.Start("SetSelection"); 

　　　　 //添加到当前选择集中  
　　　　 sel.Elements.Add(e);
 
　　　　 trans.Commit(); 
　　} 

}