执行第二个查询时出现问题 |通过 C# 在 ACCESS 中创建关系

这段代码有什么问题？

执行第二个查询时遇到刷新访问问题

OleDbConnection con = new OleDbConnection("provider=microsoft.ace.oledb.15.0; data source=" + user + ".accdb");
con.Open();
OleDbCommand comm = new OleDbCommand("alter table Mail add constraint fk_id foreign key(CID) references Bio(CID)", con);
comm.ExecuteNonQuery();
con.Close();
OleDbConnection con2 = new OleDbConnection("provider=microsoft.ace.oledb.15.0; data source=" + user + ".accdb");
con2.Open();
OleDbCommand comm2 = new OleDbCommand("alter table Num add constraint fk_id foreign key(CID) references Bio(CID)", con2);
comm2.ExecuteNonQuery();
con2.Close();