保存A实体会莫名其妙保存到B实体。这是如何回事啊！Nhibernate

 [Serializable]    public class A    {          /// <summary>        ///唯一标识        /// </summary>        public virtual string ID        {            get;            set;        }         /// <summary>        ///姓名        /// </summary>        public virtual string Name        {            get;            set;        }     }        [Serializable]    public class B    {          /// <summary>        ///唯一标识        /// </summary>        public virtual string ID        {            get;            set;        }         /// <summary>        ///姓名        /// </summary>        public virtual DateTime LastTime       {            get;            set;        }       /// <summary>        ///姓名        /// </summary>        public virtual string Name        {            get;            set;        }     }

<?xml version="1.0"  encoding="utf-8" ?><hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">    <class name="Domain.A, Domain" table="A">        <id name="ID" column="ID" type="string">                                    <generator class="uuid.hex" />          </id>                      <property name="Name"  />    <property name="LastTime"  />        </class></hibernate-mapping><?xml version="1.0"  encoding="utf-8" ?><hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">    <class name="Domain.B, Domain" table="B">        <id name="ID" column="ID" type="string">                                    <generator class="uuid.hex" />          </id>                       <property name="LastTime"  />         <property name="Name"  />        </class></hibernate-mapping>

protected void btnSave_Click(object sender, EventArgs e){   A a = new A();   a.Name = GetB();   UtilityDao.SaveOrUpdate(a);//保存A实体}private string GetB(){   string result = string.Empty;   B b = service.GetBMethod();   if(b != null)   {     result = b.Name;   }   return result;}