为什么ExecuteReader要关闭？

Question

我正在用WPF开发一个应用程序。我试图获取电子邮件，提取数据，将其保存在DB中，并在DataGrid中使用它。为了避免保存副本，我已经做了一个方法。我的问题是，我不知道为什么关闭我的dataReader？如果我在另一个.xaml类中使用它，它会正常工作，那么这里有什么问题呢？我使用过using-statement，我认为这是问题所在，但似乎并非如此。

DataReader方法：

public IEnumerable<Problem> GetAll()
{
    using (SqlConnection conn = DatabaseSingleton.GetInstance())
    {
        //conn.Open();
        using (SqlCommand command = new SqlCommand("SELECT * FROM Problem", conn))
        {
            using (SqlDataReader reader = command.ExecuteReader())
            {
                while (reader.Read())
                {
                    Problem problem = new Problem
                    {
                        Id = reader.GetInt32(0),
                        NameOfAlert = reader.GetString(1),
                        Value = Enum.Parse<Value>(reader[2].ToString()),
                        Result = Enum.Parse<Result>(reader[3].ToString()),
                        Message_Id = reader.GetString(4)
                    };
                    yield return problem;
                }
            }
        }
        //conn.Close();
    }
}

获取电子邮件的方法：

public static void MailKitLib(EmailParser emailParser)
{
    bool help = true;
    
    do
    {
        using (var client = new ImapClient())
        {
            using (var cancel = new System.Threading.CancellationTokenSource())
            {
                client.Connect(emailParser.ServerName, emailParser.Port, emailParser.IsSSLuse, cancel.Token);
                client.Authenticate(emailParser.Username, emailParser.Password, cancel.Token);
    
                var inbox = client.Inbox;
                inbox.Open(FolderAccess.ReadOnly, cancel.Token);
    
                //Here I am saving only originals
                for (int i = 0; i < inbox.Count; i++)
                {
                    var message = inbox.GetMessage(i, cancel.Token);
                    GetBodyText = message.TextBody;
                    Problem problem = new Problem(message.MessageId);
                    if (!dAOProblem.GetAll().Any(x => x.Message_Id.Equals(problem.Message_Id)))
                    {
                        dAOProblem.Save(problem);
                        Alert alert = new Alert(message.MessageId, message.Date.DateTime, message.From.ToString(), 1, problem.Id);
                        if (!dAOAlert.GetAll().Any(x => x.Id_MimeMessage.Equals(alert.Id_MimeMessage)))
                        {
                            dAOAlert.Save(alert);
                        }
                    }
                }
                //client.Disconnect(true, cancel.Token);
            }
        }
    } while (help != false);
}

更新:这正是我尝试过的。但也存在一个问题，即联系没有被关闭。

Answer 1

问题在于DatabaseSingleton.GetInstance()，它支持connection-pooling

 public IEnumerable<Problem> GetAll()
        {
            string connString = "Data Source=;Initial Catalog=;User ID=;Password=";
            using (SqlConnection conn = new SqlConnection(connString))
            {
                conn.Open();
                using (SqlCommand command = new SqlCommand("SELECT * FROM Problem", conn))
                {
                    using (SqlDataReader reader = command.ExecuteReader())
                    {
                        while (reader.Read())
                        {
                            Problem problem = new Problem
                            {
                                Id = reader.GetInt32(0),
                                NameOfAlert = reader.GetString(1),
                                Value = Enum.Parse<Value>(reader[2].ToString()),
                                Result = Enum.Parse<Result>(reader[3].ToString()),
                                Message_Id = reader.GetString(4)

                            };
                            yield return problem;
                        }
                    }
                conn.Close();
                }
            }