<?xml version="1.0" encoding="utf-8"?>
<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
  <session-factory name="EIMSData.DataObjects">
    <property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
    <property name="connection.driver_class">NHibernate.Driver.SqlClientDriver</property>
    <property name="dialect">NHibernate.Dialect.MsSql2005Dialect</property>
    <property name="connection.connection_string">Server=w2jzwzb001;Initial Catalog=EIMSM;Integrated Security=True;</property>
    <property name="show_sql">true</property>       
    <property name="use_outer_join">true</property>
    <property name="show_sql">true</property>
    <property name="proxyfactory.factory_class"> NHibernate.ByteCode.LinFu.ProxyFactoryFactory, NHibernate.ByteCode.LinFu </property>
    <property name="connection.isolation">ReadCommitted</property>
    <property name="use_proxy_validator">true</property>
  </session-factory>
</hibernate-configuration>

CREATE TABLE [dbo].[CityMaster](
   [CityCode] [int] IDENTITY(1,1) NOT NULL,
   [CityName] [varchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,
   [DateCreated] [datetime] NOT NULL,
   [DateModified] [datetime] NOT NULL,
CONSTRAINT [PK_CityMaster] PRIMARY KEY CLUSTERED
(
   [CityCode] ASC
)WITH (IGNORE_DUP_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]

using System;

namespace EIMSData.DataObjects
{
    public class City
    {
        private int _CityCode;
        private string _CityName;
        private DateTime _DateCreated;
        private DateTime _DateModified;


        public int CityCode
        {
            get
            {
                return _CityCode;
            }
            set
            {
                _CityCode = value;
            }
        }

        public string CityName
        {
            get
            {
                return _CityName;
            }
            set
            {
                _CityName = value;
            }
        }

        public DateTime DateCreated
        {
            get
            {
                return _DateCreated;
            }
            set
            {
                _DateCreated = value;
            }
        }

        public DateTime DateModified
        {
            get
            {
                return _DateModified;
            }
            set
            {
                _DateModified = value;
            }
        }
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
<class name="EIMSData.DataObjects.City, EIMSData.DataObjects" table="CityMaster" >
    <id name="Id" column="CityCode" type="System.Int32" unsaved-value="null">
        <generator class="native"/>
    </id>
    <property name="CityName" column="CityName" access="field.pascalcase-underscore" not-null="true" type="System.String"  length="50" insert="true" update="true"/>
    <property name="DateCreated" column="DateCreated" access="field.pascalcase-underscore" not-null="true" type="System.DateTime"  insert="true" update="true"/>
    <property name="DateModified" column="DateModified" access="field.pascalcase-underscore" not-null="true" type="System.DateTime"  insert="true" update="true"/>
</class>
</hibernate-mapping>

ISession session;           
            try
            {
                 session = NHibernateHelper.GetCurrentSession();
            }
            catch (Exception ex)
            {
                throw new Exception(ex.InnerException.ToString());
            }

            ITransaction tx;
            try
            {
                tx = session.BeginTransaction();
            }
            catch (Exception ex)
            {
                throw new Exception(ex.InnerException.ToString());
            }         
            City newCity = new City();
            newCity.CityName = TextBox1.Text;       
            try
            {
               session.Save(newCity); <---- Code causing error.
            }
            catch (Exception ex)
            {
                throw new Exception(ex.InnerException.ToString());
            }
           
            tx.Commit();
            NHibernateHelper.CloseSession();

protected void Application_Start(object sender, EventArgs e)
        {
            NHibernate.Cfg.Configuration  config = new NHibernate.Cfg.Configuration();
            config.Configure();
            config.AddClass(typeof(City));

            SessionFactory = config.BuildSessionFactory();
        }

public sealed class NHibernateHelper
    {
private const string CurrentSessionKey = "nhibernate.current_session";
        private static readonly ISessionFactory sessionFactory;

        static NHibernateHelper()
        {
            sessionFactory = new Configuration().Configure().BuildSessionFactory();
        }

        public static ISession GetCurrentSession()
        {
            HttpContext context = HttpContext.Current;
            ISession currentSession = context.Items[CurrentSessionKey] as ISession;

            if (currentSession == null)
            {
                currentSession = sessionFactory.OpenSession();
                context.Items[CurrentSessionKey] = currentSession;
            }

            return currentSession;
        }

        public static void CloseSession()
        {
            HttpContext context = HttpContext.Current;
            ISession currentSession = context.Items[CurrentSessionKey] as ISession;

            if (currentSession == null)
            {
                // No current session
                return;
            }

            currentSession.Close();
            context.Items.Remove(CurrentSessionKey);
        }

        public static void CloseSessionFactory()
        {
            if (sessionFactory != null)
            {
                sessionFactory.Close();
            }
        }
    }