为什么.net 把*.cs做成dll引用时，dll里面不能用xmldocument类？

为什么.net 把*.cs做成dll引用时，dll里面不能用xmldocument类？如果有则报错：Error 2 The type 'System.Xml.XmlDocument' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Xml, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'. F:\li\dllMobile\3\Mobile\Mobile\Login.cs 53 12 Mobile
如果不做成dll是可以运行的，我也加了using System.xml;