Close the reader.
if (clas == "firstac")
{
string query = "select firstac from vacseat where tno='" + tno + "'";
SqlCommand cmd2 = new SqlCommand(query, conn);
SqlDataReader dr = cmd2.ExecuteReader();
if (dr.Read())
{
seat = int.Parse(dr["firstac"].ToString());
}
dr.Close();
}