Nhibernate 配置文件问题

我在项目中使用app.config来配置nhibernate时测试正常
其中内容为:

<?xml version="1.0" encoding="utf-8"?>

<configuration>

  <configSections>

    <section name="hibernate-configuration" type="NHibernate.Cfg.ConfigurationSectionHandler, NHibernate" />

  </configSections>

  <hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">

    <session-factory>

      <property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>

      <property name="dialect">NHibernate.Dialect.MsSql2005Dialect</property>

      <property name="connection.driver_class">NHibernate.Driver.SqlClientDriver</property>

      <property name="connection.connection_string">Data Source=1AC425E4D0B74F1;Initial Catalog=GuestBook;User ID=sa;Password=1</property>

      <mapping assembly="NhibernateSample.Model"/>

    </session-factory>

  </hibernate-configuration>

</configuration>

但是我看见很多直接使用的是"hibernate.cfg.xml"文件而不是app.config
于是我把app.config文件修改成hibernate.cfg.xml,并且也已经把该文件的输出选项设置为了“始终复制”

<?xml version="1.0" encoding="utf-8"?>

<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">

  <session-factory>

    <property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>

    <property name="dialect">NHibernate.Dialect.MsSql2005Dialect</property>

    <property name="connection.driver_class">NHibernate.Driver.SqlClientDriver</property>

    <property name="connection.connection_string">Data Source=1AC425E4D0B74F1;Initial Catalog=GuestBook;User ID=sa;Password=1</property>

    <mapping assembly="NhibernateSample.Model"/>

  </session-factory>

</hibernate-configuration>

结果测试的时候一直显示的是：
NhibernateSample.Test.LeaveMessageDALTest.InsertTest:
NHibernate.MappingException : Could not compile the mapping document: NhibernateSample.Model.HbmMaps.Users.hbm.xml
----> System.InvalidOperationException : Could not find the dialect in the configuration

不知是何故，请大家帮忙。