如何使用c＃从数据库获取Max id？

Question

cmd = new OleDbCommand("SELECT max(substr(tr_refno,9,6))as REFID from ECHALLAN WHERE DEPT='" +tmpDept.ToString() + "' and substr(tr_refno,5,2) ='" + Tmpmonth + "'", con);                         
maxid = Convert.ToInt16( cmd.ExecuteScalar());

  

错误= InvalidCastException未被用户代码

处理

Answer 1

在转换之前，您可以检查无效：

var value = cmd.ExecuteScalar();
int maxI;
If(value !=null)
   maxId = Convert.ToInt32(value);
else 
//......................

Answer 2

尝试此命令：

cmd = new OleDbCommand("SELECT Max(COALESCE(substr(tr_refno,9,6),0)) as REFID from ECHALLAN WHERE DEPT='" +tmpDept.ToString() + "' and substr(tr_refno,5,2) ='" + Tmpmonth + "'", con);