错误System.NullReferenceException

Question

hi guys when ever i try to execute the project it appears this error when i am trying to store the objects. as you can see this is the pickup form which will allow user to store the value in different classes. there are 3 different class the value will be store at.. you see am still a newbess at the c# thing. i have customer class which will store the details of the customer, pickup class with details of the pickup and same with delivery. and with the other class Visit will only store when the time is occured. i don't know if that is the problem.. let me know if you want to see more code.. thank you

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

You're accessing properties of thePickup , but at no point have you assigned an object to the variable. So thePickup is still null , which is why you get the NullReferenceException.

At some point you need to instatiate a Pickup object and assign it to thePickup like so:

thePickup = new Pickup();

You should probably do this in either the constructor or the PickupForm_Load event handler.

The same goes for theVisit and 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;   
}

You need to initialize respective properties