错误:无法将类型'string'隐式转换为'int?'

大家好，
我正在尝试构建以下类，因为无法在类this.id = e.id上得到错误，因为无法将类型``string''隐式转换为``int?''.

非常感谢，
车丹.

Hey guys,
I am trying yo build the following class for which i get an error on the line this.id = e.id as cannot implicitly convert type ''string'' to ''int?''.

many many Thanks,
Chetan.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Data.Linq;
using System.Text;
using System.Runtime.Serialization;
using HsInternet.Data.DAL;

namespace HsInternet.Data.BLL
{
    public class Baby
    {
        public int id { get; set; }
        public string name { get; set; }
        public string parents { get; set; }
        public int gender { get; set; }
        public string weight { get; set; }
        public string length { get; set; }
        public DateTime date { get; set; }
        public DateTime time { get; set; }
        public string image { get; set; }
        public string ipa { get; set; }
        public string ipb { get; set; }


        public Baby()
        {
            this.id = -1;
            this.name = string.Empty;
            this.parents = string.Empty;
            this.gender = -1;
            this.weight = string.Empty;
            this.length = string.Empty;
            this.date = DateTime.Now;
            this.time = DateTime.Now;
            this.image = string.Empty;
            this.ipa = string.Empty;
            this.ipb = string.Empty;
        }

        public Baby(Data.DAL.Baby e)
        {
            this.id = e.id;     //getting error on this line
            this.name = e.name;
            this.parents = e.parents;
            this.gender = e.gender;
            this.weight = e.weight;
            this.length = e.length;
            this.date = e.date;
            this.time = e.time;
            this.image = e.image;
            this.ipa = e.ipa;
            this.ipb = e.ipb;
        }

        public static void Parse(DAL.Baby e, Baby t)
        {
            e.id = t.id;