Question

我是这种模式的新手，我想知道为什么我会收到错误

“无法将dbnull转换为其他类型”

我知道我的代码不起作用，这就是为什么它会在_view上返回一些空值，任何帮助？我得到这个在线模型.CustomerID = _view.CustomerID;

  Model
  public class CustomerModel
  {
            private string _customerid;
            public string CustomerID;
            {
                get { return _customerid; }
                set { _customerid = value; }
            }
            private string _orderstatus;
            public string OrderStatus
            {
                get { return _orderstatus; }
                set { _orderstatus = value; }
            }
            public void UpdateOrderStatus()
            {
                var connSettings = ConfigurationManager.ConnectionStrings["DB"];
                {
                    string CN = connSettings.ConnectionString;
                    MySqlConnection conn = new MySqlConnection(CN);
                    MySqlCommand cmd = new MySqlCommand("update customerorders set order_status = @value where customerid = @customerid", conn);
                    conn.Open();
                    cmd.Parameters.AddWithValue("@customerid", _customerid);
                    cmd.Parameters.AddWithValue("@value", _orderstatus);
                    cmd.ExecuteNonQuery();
                }
            }
     }

  Interface
  public interface ICustomer
  {
        void SetPresenter(CustomerPresenter presenter);
        void UpdateCustomerOrderStatus(CustomerModel record);
        string CustomerID { get; set; }
        string OrderStatus { get; set; }
  }

  Presenter
  public class CustomerPresenter
  {
        ICustomer _view;
        CustomerModel _model = new CustomerModel();

        public void CustomerPresenter(ICustomer view)
        {
           _view = view;
        }
        private void updateCustomerOrderStatus(CustomerModel model)
        {
            model.CustomerID = _view.CustomerID;
            model.OrderStatus = _view.OrderStatus;
        }
        public void Save()
        {
            updateCustomerOrderStatus(CustomerModel model)
            this._view.UpdateCustomerOrderStatus(CustomerModel record);
        }
  }

  CustomerForm
  public partial class frmCustomer: Form, ICustomer
  {
        CustomerPresenter _presenter
        public void SetPresenter(CustomerPresenter presenter)
        {
           _presenter = presenter
        }
        public string CustomerID
        {
        get { return this.txtCustomerID.Text; }
        set { txtCustomerID.Text = value; }
        }
        public string OrderStatus
        {
        get { return this.txtOrderStatus.Text; }
        set { txtOrderStatus.Text = value; }
        }
        public void UpdateCustomerOrderStatus(CustomerModel record)
        {
            record.CustomerID = txtCustomerID.Text;
            record.OrderStatus = txtOrderStatus.Text;
        }
        private void btnSave_Click(object sender, EventArgs e)
        {
            _presenter.Save();
        }
  }

Answer 1

您的_view现已确实已正确初始化，但_view.CustomerID始终为null，因为您未在get和set内执行任何操作。因此，当您尝试使用get时，它不会返回任何内容，这就是您获得此异常的原因。

Answer 2

根据代码，_view永远不会被初始化。

Answer 3

您是否将ViewID隐藏在视图中，如果不将其隐藏在视图中，我认为它将修复错误

使用MVP模式更新MySQL数据库

3 个答案: