string.IsNullOrEmpty

 if (string.IsNullOrEmpty(entry.Idaccount.ToString()))
            {
                cmdText = "UPDATE SM_FC_TransactionEntry SET Idaccount=null WHERE ID='{0}'";
                cmdText = string.Format(cmdText, entry.ID.ToString());
            }

string cmdText = @"select TransactionEntry.mergecode as mergecode,TransactionAuxiliaryInfo.idvouchertype as idvouchertype from SM_FC_TransactionEntry as TransactionEntry left join SM_FC_TransactionAuxiliaryInfo as TransactionAuxiliaryInfo on TransactionAuxiliaryInfo.id=TransactionEntry.idauxiliaryinfo where mergecode='{0}' and idvoucherType<>'{1}'"; cmdText = string.Format(cmdText, mergeCode, idvoucherType);