FNH.cs

public class MyClassMap: ClassMap<MyClass>
{
    public MyClassMap()
    {
        Id(x => x.Id)
          .Column("MyClassId")
          .GeneratedBy.HiLo("100");

        Map(x => x.Something)
          .Length(150);
    }
}

NH3.2.cs

public class MyClassMap: ClassMapping<MyClass>
{
    public MyClassMap()
    {
        Id(x => x.Id, map =>
        {
            map.Column("MyClassId");
            map.Generator(Generators.HighLow, gmap => gmap.Params(new { max_low = 100 }));
        });
        Property(x => x.Something, map => map.Length(150));
    }
}