求大佬帮忙，C#中，插入语句时，如何把datetime的默认值 1990-1-1 取消，变成空，数据库已设置允许为空

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本文探讨了在C#中如何将实体类中DateTime字段的默认值从1990-1-1更改为NULL，特别是在数据库允许空值的情况下。通过修改实体类属性并调整插入语句，详细介绍了如何正确实现这一需求。

求大佬帮忙，C#中，插入语句时，如何把datetime的默认值 1990-1-1 取消，变成空，数据库已设置允许为空

[Column(“REGIST_DATE”)]
public DateTime? Regist_date { get; set; }

这个是实体类的日期字段

QC_CompanyEntity entity = strEntity.ToObject<QC_CompanyEntity>();

     这个是插入语句   
        //var db = this.BaseRepository("BaseDb").BeginTrans();
        try
        {
            if (!string.IsNullOrEmpty(keyValue))
            {
                UserInfo userInfo = LoginUserInfo.Get();
                entity.ZBID = userInfo.companyId;
                entity.F_CreateDate = DateTime.Now;
                var dp = new DynamicParameters(new { });
                string strSql = "UPDATE  QC_Company SET   ZBID='" + entity.ZBID + "',  Name='" + entity.Name + "',P_Code='" + entity.P_Code + "', B_Code='" + entity.B_Code + "', F_Code='" + entity.F_Code + "', Ammeter_no='" + entity.Ammeter_no + "', Watermeter_no='" + entity.Watermeter_no + "', TYSHXYDM='" + entity.TYSHXYDM + "', ZCZB='" + entity.ZCZB + "', ZCZBBZMY='" + entity.ZCZBBZMY + "', NWZ='" + entity.NWZ + "',  Regist_date='" + entity.Regist_date + "',TZFJCZBL='" + entity.TZFJCZBL + "', ZCDZ='" + entity.ZCDZ + "', LXR='" + entity.LXR + "', LXDH='" + entity.LXDH + "', Email='" + entity.Email + "',  FRXM='" + entity.FRXM + "', FRLXDH='" + entity.FRLXDH + "', QYBGDH='" + entity.QYBGDH + "', ZJL='" + entity.ZJL + "', ZJLLXDH='" + entity.ZJLLXDH + "', BGRYRS='" + entity.BGRYRS + "', SQYFMJ='" + entity.SQYFMJ + "', HYBM='" + entity.HYBM + "', RYSJ='" + entity.RYSJ + "', DJBM='" + entity.DJBM + "', YYQK='" + entity.YYQK + "', SNDXSSR='" + entity.SNDXSSR + "', SNDZCZ='" + entity.SNDZCZ + "', SNDLS='" + entity.SNDLS + "', SNDCYRS='" + entity.SNDCYRS + "', SNDRXRCJHQK='" + entity.SNDRXRCJHQK + "', JYFW='" + entity.JYFW + "', GXJSQYRDSJ='" + entity.GXJSQYRDSJ + "', KJXZXQYRDSJ='" + entity.KJXZXQYRDSJ + "', SJDWZCZBJ='" + entity.SJDWZCZBJ + "', SSQK='" + entity.SSQK + "',F_CreateDate='" + entity.F_CreateDate + "' WHERE  ID='" + keyValue + "'";
                
                 this.BaseRepository().ExecuteBySql(strSql);

麻烦大佬说得清楚一下，新手