SqlDataReader 出现问题

ArrayList userids = new ArrayList();
while(MyReader.Read())
userids.add(MyReader.GetInt32(0));
MyReader.Close();
for(int i = 0;i < userid.Count; i++)
{
int userid = userids[i];
string sqlQueryUser = "select userid,username,truename,unitname,telnumber,email from res_user where userid='" + userid + " '";
comm = new SqlCommand(sqlQueryUser, conn);
SqlDataAdapter da = new SqlDataAdapter(comm);
da.Fill(dset,"maintainusers");
}

把MyReader.Close();
放到da.Fill(dset,"maintainusers"); 后面啦..!!

comm = new SqlCommand(sqlQueryUser, conn);
SqlDataAdapter da = new SqlDataAdapter(comm);
da.Fill(dset,"maintainusers");

调用da.Fill的时候会conn.open()此时因为之前已经打开了，所以报错！