Nhibernate：无法编译映射文档.hbm.xml

//xml file

// xml文件

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   namespace="ContactDomain"
                   assembly="ContactDomain">

  <class name="Contact" table="tbl_contact">

    <id name="ContactId" column="contact_id" type="int">
      <generator class="identity"></generator>
    </id>

    <property name="FirstName"  column="first_name" type="String"/>
    <property name="LastName"   column="last_name"  type="String"/>
    <property name="Email"      column="email"      type="String"/>
    <property name="Telephone"  column="telephone"  type="String"/>
  </class>
</hibernate-mapping>
<?xml version="1.0" enc