数据库更新语句问题
运行以下代码，出现 System.Data.SqlClient.SqlException: 列名 'daoqi' 无效。我用红字标明了出错的位置。请专家指教！


....
int userid = Convert.ToInt32(Session["userid"]); 
string sql2 = "select adddate,LimitYear,loginnum from users where code='" + Number + "'";
DataSet ds2 = bc.getDataSet(sql2); DateTime addTime = Convert.ToDateTime(ds2.Tables[0].Rows[0][0]);
int liyear = Convert.ToInt32(ds2.Tables[0].Rows[0][1]);
DateTime daoqi = Convert.ToDateTime(addTime.AddYears(+liyear));

if (DateTime.Now > daoqi)
   {
       Response.Redirect("index.aspx");
    }

string sqlc2 = "update users set LimitDate= daoqi where userid=" + userid;
//以下这一句是出问题的语句
bc.executesql(sqlc2);


....
public void executesql(string sql)
    {
        SqlConnection conn = Getconn();
        try
        {
            conn.Open();
            SqlCommand cmd = conn.CreateCommand();
            cmd.CommandText = sql;
            cmd.ExecuteNonQuery();
        }
        catch (Exception e)
        {
           //在这里抛出异常 
           throw e;
        }
        finally
        {
            conn.Close();
            conn.Dispose();
        }
    }