How to get Max id From Database using c#?

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());

Error= InvalidCastException was unhandled by user code

Answer 1

Before Converting it you could check the nullity:

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

Answer 2

试试这个命令：

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);