类型或命名空间名称'；Int32'；未能找到(是否缺少using指令或程序集引用?)

我的代码没有编译，出现错误：

找不到类型或命名空间名称"Int32"（是否缺少using指令或程序集引用？）

为什么会发生这种情况/我该如何解决？

这是有问题的代码：

///***********************************************************
///Author Name: Harkamal Singh
///Creation Date: 17th Nov, 2008
///File Name: CountryPrp.cs         Component Used: 
///Called From: Business Logic Layer  
///Description: Class File For Booking Functionality
///Tables Accessed: 
///Program specs:
///UTP doc:
///Tested By:
///***********************************************************************
///Modification History:
///Change No.   Changed By  Date    Version Raised By/SRS No    Description
///***********************************************************************using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using System.Collections;
using System.Collections.Generic;

/// <summary>
/// Summary description for CountryPrp
/// </summary>
namespace BLL
{
public class CountryPrp
{
    public CountryPrp()
{
        //
        // TODO: Add constructor logic here
        //
    }
    #region  tCountryPropertyClass
        private Int32 iCountryId;
        private String sCountryName;
        private DateTime dtCreated;
        private DateTime dtModified;
        private Int32 iCreatedBy;
        private Int32 iModifiedBy;
        private Boolean bisActive;
        private Char sOperationType;
        private Int16 iReturnid;

        public Int32 p_iCountryId
        {
            get
            {
                return iCountryId;
            }
            set
            {
                iCountryId = value;
            }
        }
        public String p_sCountryName
        {
            get
            {
                return sCountryName;
            }
            set
            {
                sCountryName = value;
            }
        }
        public DateTime p_dtCreated
        {
            get
            {
                return dtCreated;
            }
            set
            {
                dtCreated = value;
            }
        }
        public DateTime p_dtModified
        {
            get
            {
                return dtModified;
            }
            set
            {
                dtModified = value;
            }
        }
        public Int32 p_iCreatedBy
        {
            get
            {
                return iCreatedBy;
            }
            set
            {
                iCreatedBy = value;
            }
        }
        public Int32 p_iModifiedBy
        {
            get
            {
                return iModifiedBy;
            }
            set
            {
                iModifiedBy = value;
            }
        }
        public Boolean p_bisActive
        {
            get
            {
                return bisActive;
            }
            set
            {
                bisActive = value;
            }
        }
        public Char p_sOperationType
        {
            get
            {
                return sOperationType;
            }
            set
            {
                sOperationType = value;
            }
        }
        public Int16 p_iReturnid
        {
            get
            {
                return iReturnid;
            }
            set
            {
                iReturnid = value;
          }
        }

  #endregion
}

///***********************************************************************using System;

///***********************************************************************
using System;