未将对象引用设置到对象的实例

Question

当我使用满300减100时，显示报错，找不到是哪里出了问题，求大佬指点

 

winform窗体代码

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;
using WindowsFormsApp4.MyClasses;

namespace WindowsFormsApp4
{
    public partial class Form1 : Form
    {
        //用于存放“用户添加的”商品信息的集合

        Dictionary<string, Good> goods = new Dictionary<string, Good>();

        //用于存放商品下拉框商品名称和对应的价格的集合

        Dictionary<string,double> jh=new Dictionary<string, double>();

        public Form1()
        {
            InitializeComponent();
            //为jh集合添加商品名称和价格的键值对
            jh.Add("湘西腊肉", 45.00);
            jh.Add("东江鱼", 42.00);
            jh.Add("长沙臭豆腐", 5.00);
            jh.Add("临武鸭", 90.00);
            //在下拉框中显示商品名称
            cbb_name.Items.AddRange(jh.Keys.ToArray());//取出jh中所有键，并转换为数组
        }

        private void cbb_name_SelectedIndexChanged(object sender, EventArgs e)
        {
           
            //获取下拉框选中的商品名称
            string goodName = cbb_name.Text;
            //将商品名称作为键在names集合中找到对应的值
            txt_price.Text = jh[goodName].ToString("0.00"); //取出的值为double类型，需要转化为字符串，0.00为显示为小数点后两位
        }

        private void cbb_jffs_SelectedValueChanged(object sender, EventArgs e)
        {
            if (this.cbb_jffs.Text == "打折")
            {
                this.txt_zkou.Enabled = true;
            }
            else
            {
                this.txt_zkou.Enabled = false;
                this.txt_zkou.Text = "1.0";
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (goods.ContainsKey(cbb_name.Text))
            {
                MessageBox.Show("已添加相同名称的商品");
                return;
            }
            Good good = new Good();
            good.Name = cbb_name.Text;
            good.Price = double.Parse(txt_price.Text);
            good.Number = int.Parse(txt_number.Text);
            good.Casname = cbb_jffs.Text;
            Cash cash = null;
            switch (cbb_jffs.Text)
            {
                case "正常收费":
                    cash = new NormalCash();
                    break;
                case "打折":
                    cash = new RataCash();
                    break;
                case "满300减100":
                    cash = new ReturnCash();
                    break;
            }
            good.TotalPrice = cash.CalPrice(good.Price * good.Number, double.Parse(txt_zkou.Text));
            //将对象添加到集合
            goods.Add(good.Name, good);
            this.dataGridView1.DataSource = null;
            this.dataGridView1.DataSource = goods.Values.ToList();//将字典集合goods中所有值取出，并转换为列表集合
        }

        private void button2_Click(object sender, EventArgs e)
        {
            //删除,需要将dataGridView控件的selectedMode属性设置为fullrowSelect(整行选中)
            //①获取用户选中的行的第一格数据(商品名称)
            string goodName = this.dataGridView1.SelectedRows[0].Cells[0].Value.ToString();
            //②按获取的商品名称删除集合数据
            goods.Remove(goodName);
            //③刷新数据
            this.dataGridView1.DataSource = null;
            this.dataGridView1.DataSource = goods.Values.ToList();//将字典集合goods中所有值取出，并转换为列表集合
        }

        
    }
}

 Cash

abstract class Cash //优惠方式的抽象类
    {
        public abstract double CalPrice(double oldPrice, double rate);
    }

Good

public class Good
    {
        public string Name { get; set; }
        public double Price { get; set; }
        public int Number { get; set; }
        public string Casname { get; set; }//计费（优惠）方式总称
        public double TotalPrice  { get; set; }//单价*数量的结果优惠后的总价
    }

NormalCash

class NormalCash:Cash
    {
        //重写抽象类的方法
        public override double CalPrice(double oldPrice, double rate)
        {
            return oldPrice;
        }
    }

RataCash

class RataCash:Cash
    {
        
        public override double CalPrice(double oldPrice, double rate)
        {
            return oldPrice * rate;//原价*折扣率
        }
    }

ReturnCash

class ReturnCash:Cash
    {
        
        public override double CalPrice(double oldPrice, double rate)
        {
            if (oldPrice >= 300)
            {
                return oldPrice - 100;//返利100
            }
            return oldPrice;//返回原值
        }
    }

Answer 1

找了好几遍，满300减100和满300返100