nhibernate GetCurrentSession错误

Question

ISessionFactory sessionFactory =  
                new Configuration().Configure().BuildSessionFactory();
var session = sessionFactory.GetCurrentSession();

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在该代码中，它抛出错误消息“No CurrentSessionContext configured（设置属性current_session_context_class）！”

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关于nhibernate的app.config如下：

<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
<session-factory>
  <property name="dialect">NHibernate.Dialect.MySQL5Dialect</property>
  <property   name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
  <property name="connection.connection_string">
    Server=localhost;initial catalog=test;uid=root;pwd=root123
  </property>
  <property name="current_session_context_class">thread</property>
  <mapping assembly="testNH" />
</session-factory>

我的错误在哪里？以及如何修改代码？