◆◆◆◆◆指定编码格式的问题，高手请进◆◆◆◆◆

StringBuilder sb = new StringBuilder();
StringWriter writer = new StringWriter(sb);
Type [] types = new Type[4];
types[0] = obj1.GetType();
types[1] = obj2.GetType();
types[2] = obj3.GetType();
types[3] = obj4.GetType();

XmlSerializer ser = new XmlSerializer(ob.GetType(), types);
ser.Serialize(writer, ob);
writer.Close();
我如何指定writer的编码格式，因为序列化到sb中的xml文档的开头编码写入的是“utf-16",我想是shift－jis，如何处理，高手指点一下。