create table MyTest (Id int identity, Field1 nvarchar( 50 ), Field2 nvarchar( 50 ))

public class BaseClass
{
   public virtual object Id { get; set; }
   public virtual string Field1 { get; set; }
}
public class NestedClass : BaseClass
{
   public virtual string Field2 { get; set; }
}

<!-- BaseClass -->
<class name="BaseClass" table="[dbo].[MyTest]">
   <id name="Id" column="Id" type="Int32"><generator class="native"/></id>
   <property name="Field1" column="Field1" type="string"/>
</class>
<!-- NestedClass -->
<class name="NestedClass" table="[dbo].[MyTest]">
   <id name="Id" column="Id" type="Int32"><generator class="native"/></id>
   <property name="Field1" column="Field1" type="string"/>
   <property name="Field2" column="Field2" type="string"/>
</class>

ISession session;
var listBaseClass = session.CreateCriteria(typeof(BaseClass)).List<BaseClass>();
var listNestedClass = session.CreateCriteria(typeof(NestedClass)).List<NestedClass>();

--Load BaseClass
select Id, Field1, Field2 from MyTest
select Id, Field1 from MyTest
--Load NestedClass:
select Id, Field1, Field2 from MyTest

--Load BaseClass
select Id, Field1 from MyTest
--Load NestedClass:
select Id, Field1, Field2 from MyTest

Id | Field1 | Field2
--------------------
1  | 11     | 12
2  | 21     | 22
3  | 31     | 32

var listBaseClass = session.CreateCriteria(typeof(BaseClass)).List<BaseClass>();

select Id, Field1 from MyTest

var listNestedClass = session.CreateCriteria(typeof(NestedClass)).List<NestedClass>();

select Id, Field1, Field2 from MyTest