这个问题，整了我一个下午

public abstract class A<T> : B where T : C {
        //protected
        public A(T importer)
            : base(importer) {
        }
        //internal
        public virtual new T Importer { get { return (T)base.Importer; } }
          public  abstract ElementHandlerTable<T> ElementHandlerTable { get; }
        public override void ProcessElementOpen(XmlReader reader) {
        }
        public override void ProcessElementClose(XmlReader reader) {
        }
        public override bool ProcessText(XmlReader reader) {
            return true;
        }
        //  public 
        public override B C(XmlReader reader) {
            string localName = reader.LocalName;
            ElementHandler<T> handler;
            if (ElementHandlerTable.TryGetValue(localName, out handler))
                return handler(Importer, reader);
            else
                return null;
        }
    }