NHibernate异常"Found shared references to a collection"

场景：file和attachment是多对多的关系，存在一张关联表file和attachment的关联表。

一个file有多个attachment，在file的实体类中应该有一个list保存该file的所有attachment。

如下：

private IList<RsAttachment> _attachments = new List<RsAttachment>();public virtual IList<RsAttachment> Attachments        {            get { return _attachments; }            set { _attachments = value; }        }

映射文件部分如下：

<?xml version="1.0" encoding="utf-8" ?><hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="RimaSoftApplication.Domain" assembly="RimaSoftApplication.Domain">  <class name="RimaSoftApplication.Domain.RsFiles" table="T_Files" lazy="false">    <id name="ID" column="FileID">      <generator class="native" />    </id>    <property name="PublicFileBoxID" column="PublicFileBoxID" />    <property name="PrivateFileBoxID" column="PrivateFileBoxID" />    <property name="KeyWords" column="KeyWords" />    <property name="FileTitle" column="FileTitle" />    <property name="FileContent" column="FileContent" />    <property name="CreateTime" column="CreateTime" />    <property name="Readers" column="Readers" />    <property name="Remark" column="Remark" />    <many-to-one name="CreateUser" column="CreateUserID" class="RsUser" />    <!--<many-to-one name="PrivateFileBox" column="FileBoxID" class="RsPrivateFileBox" />    <many-to-one name="PublicFileBox" column="FileBoxID" class="RsPublicFileBox" />-->    <bag name="Attachments" table="T_Files_Attachment" lazy="true">      <key column="FileID"/>      <many-to-many class="RimaSoftApplication.Domain.RsAttachment" column="AttachmentID"/>    </bag>  </class></hibernate-mapping>

当需要复制一份文件时，需要同时复制它所有的attachment，如果这样拷贝：

 public CopyOrCutFileResponse CopyFile(CopyFileRequest request)        {            CopyOrCutFileResponse response = new CopyOrCutFileResponse();            if (request != null)            {                RsFiles rsFile = _filesDao.GetById(request.ID);                RsFiles rsFileNew = new RsFiles();                // 对新文件进行赋值                rsFileNew.CreateTime = rsFile.CreateTime;                rsFileNew.CreateUser = rsFile.CreateUser;                rsFileNew.FileContent = rsFile.FileContent;                rsFileNew.FileTitle = rsFile.FileTitle;                rsFileNew.KeyWords = rsFile.KeyWords;                rsFileNew.PrivateFileBoxID = request.destFileBoxID;  //所属PrivateFileBox的ID                rsFileNew.Readers = rsFile.Readers;                rsFileNew.Remark = rsFile.Remark;                rsFileNew.Attachments = rsFile.Attachments;                _filesDao.Add(rsFileNew);                response.OPResult = SysEnum.OperationResult.OR_SUCCEESS;            }            return response;        }

则会出现题目中的错误，因为这两个对象引用的list是同一个list,这在hibernate中是不允许的，参见Hibernate reference第6章的"Two entities may not share a reference to the same collection instance"。
这种问题常见于复制对象时。

正确的复制应该如下所示：传值而不是传递引用！！

 public CopyOrCutFileResponse CopyFile(CopyFileRequest request)        {            CopyOrCutFileResponse response = new CopyOrCutFileResponse();            if (request != null)            {                RsFiles rsFile = _filesDao.GetById(request.ID);                RsFiles rsFileNew = new RsFiles();                // 对新文件进行赋值                rsFileNew.CreateTime = rsFile.CreateTime;                rsFileNew.CreateUser = rsFile.CreateUser;                rsFileNew.FileContent = rsFile.FileContent;                rsFileNew.FileTitle = rsFile.FileTitle;                rsFileNew.KeyWords = rsFile.KeyWords;                rsFileNew.PrivateFileBoxID = request.destFileBoxID;  //所属PrivateFileBox的ID                rsFileNew.Readers = rsFile.Readers;                rsFileNew.Remark = rsFile.Remark;                foreach (RsAttachment item in rsFile.Attachments)                {                    rsFileNew.Attachments.Add(item);                }                _filesDao.Add(rsFileNew);                response.OPResult = SysEnum.OperationResult.OR_SUCCEESS;            }            return response;        }