public T this[string key]
 {
     get
     {
         return(_items[_keys.IndexOf(key)]);
     }
     set
     {
         _items[_keys.IndexOf(key)] = value;
     }
 }
 public virtual int IndexOf(T item)
 {
     return(_items.IndexOf(item));
 }
 public virtual int IndexOf(T item)
 {
     return(_coll.IndexOf(item));
 }