錯誤System.NullReferenceException

Question

嗨，大家好，當我嘗試執行項目時，當我嘗試存儲對象時就會出現此錯誤。 如您所見，這是取貨表格，允許用戶將值存儲在不同的類中。 值將存儲在3個不同的類中。.您會發現c＃仍然是新手。 我有客戶類別，該類別將存儲客戶的詳細信息，取件類別以及取件的詳細信息，以及交貨信息。 與其他類一起使用，Visitor將僅在發生時間時存儲。 我不知道這是否是問題..讓我知道您是否想查看更多代碼..謝謝

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;


namespace coursework2
{

    public partial class PickupForm : Form
    {
        private MainForm mainform;
        private Pickup thePickup;
        private Delivery theDelivery;
        private Visit theVisit;


      public Pickup pickup
        {
            get { return thePickup;}
            set { thePickup = value;}
        }
      public Delivery delivery
      {
          get { return theDelivery; }
          set { theDelivery = value; }
      }

      public Visit visit
      {
          get { return theVisit; }
          set { theVisit = value; }

      }

        public PickupForm()
        {
            InitializeComponent();
        }

        /*public MainForm ParentForm
        {
            get { return mainform; }
            set { mainform = value; }
        }*/

        private void PickupForm_Load(object sender, EventArgs e)
        {
           if (thePickup != null)
            {
                textCname.Text = thePickup.PickupName;
                textAddress.Text = thePickup.PickupAddress;
                textDate.Text = theVisit.DateTime.ToString();
                textDname.Text = theDelivery.DeliveryName;
                textDaddress.Text = theDelivery.DeliveryAddress;
            }
        }

        private void btnReturn_Click(object sender, EventArgs e)
        {

            this.Close();
            /*mainform.Show();*/
        }


        private void btnClear_Click(object sender, EventArgs e)
        {
            textCname.Clear();
            textAddress.Clear();
            textDate.Clear();
            textDname.Clear();
            textDaddress.Clear();
        }

        private void btnPickup_Click(object sender, EventArgs e)
        {
            thePickup.PickupName = textCname.Text; //error occurs from this line
            thePickup.PickupName = textAddress.Text;
            theVisit.DateTime = DateTime.Parse(textDate.Text);
            theDelivery.DeliveryName = textDname.Text;
            theDelivery.DeliveryAddress = textDaddress.Text;



            this.Close();
        }




        private void textDate_TextChanged(object sender, EventArgs e)
        {

        }

        private void textCname_TextChanged(object sender, EventArgs e)
        {

        }
    }
}

Answer 1

您正在訪問thePickup屬性，但thePickup沒有為該變量分配對象。 所以thePickup仍然為null ，這就是為什么您得到NullReferenceException的原因。

在某些時候，您需要實例化一個Pickup對象並將其分配給thePickup如下所示：

thePickup = new Pickup();

您可能應該在構造函數或PickupForm_Load事件處理程序中執行此操作。

theVisit和theDelivery 。

Answer 2

private void PickupForm_Load(object sender, EventArgs e)
{
    if(thePickup == null)
    {
        thePickup = new Pickup();
    }

    if(theDelivery == null)
    {
        theDelivery = new Delivery();
    }

    if(theVisit == null)
    {
        theVisit =  new Visit();
    }

    textCname.Text = thePickup.PickupName;
    textAddress.Text = thePickup.PickupAddress;
    textDate.Text = theVisit.DateTime.ToString();
    textDname.Text = theDelivery.DeliveryName;
    textDaddress.Text = theDelivery.DeliveryAddress;   
}

您需要初始化各自的屬性