對象不能從DBNull 轉換為其他類型的解決方案

string sql_str="select MAX(lID) from liuyan":改成

語句string sql_str = "select iif((MAX(lID)) is null,'0','1') from liuyan";

iff(判斷，是，否)

但是只能添加一次，因為第二次添加運行string sql_str = "select iif((MAX(lID)) is null,'0','1') from liuyan";
           int lid=DBHelper.GetScalar(sql_str)+1;

會將lid設定為2，所以以後添加的lID都是2，那樣後台操作的時候會一起刪了lID=2的

所以我修改了代碼：

如My Code

 public static void AddMsg(Msg msg)
       {

           string sql_str = "select iif(lID is null,'0','1') from liuyan";
           int lid = DBHelper.GetScalar(sql_str);
           if (lid == 0)
           {
           string sql2 = "insert into liuyan(lID) values("+lid+")";
           DBHelper.ExecuteCommand(sql2);
           }
           else
           {
               string sql1 = "select MAX(lID) from liuyan";
               int lid2 = DBHelper.GetScalar(sql1)+1;
               string sql = "insert into liuyan(lID,lName,lTel,lEmail,lMsg) values(" + lid2 + ",@lName,@lTel,@lEmail,@lMsg)";
               OleDbParameter[] para = new OleDbParameter[]
           {
               new OleDbParameter("@lName",msg.LName),
               new OleDbParameter("@lTel",msg.LTel),
               new OleDbParameter("@lEmail",msg.LEmail),
               new OleDbParameter("@lMsg",msg.LMsg),
           };
               DBHelper.ExecuteCommand(sql, para);
           }

              
       }

 

呵呵，完成，看了網上的，千篇一律，不知道都拷哪裡的，不知道到底是誰寫的，一個字不差的都拷貝拷貝再拷貝，本人實在看不下去，於是自己實踐寫下筆記與大家分享。