* .csv文件保存-'System.NullReferenceException'C＃

Question

我有一个任务来创建一个程序，该程序可以读取并保存更改的* .csv文件。因此，经过2周的构建，学习和尝试，我得到了下一个无法正常运行的代码。它正在阅读Okay，但我在“保存”按钮上苦苦挣扎。它具有“ System.NullReferenceException”错误。 //writer.Write（“，” + dataGridView1.Rows [i] .Cells [j] .Value.ToString（））; -这是一行错误。error

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 System.IO;

namespace ITApp
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

            DataTable table = new DataTable();

        private void Form1_Load(object sender, EventArgs e)
        {
          //header names
            table.Columns.Add("Item Code", typeof(string));
            table.Columns.Add("Item Description", typeof(string));
            table.Columns.Add("Item Count", typeof(int));
            table.Columns.Add("On Order", typeof(string));

            dataGridView1.DataSource = table;
        }
//read file
        private void btnOpen_Click(object sender, EventArgs e)
        {
            string[] lines = File.ReadAllLines(@"C:\Stockfile\stocklist.csv");
            string[] values;

            for(int i = 1; i < lines.Length; i++)
            {
                values = lines[i].ToString().Split(',');
                string[] row = new string[values.Length];

                for (int j = 0; j < values.Length; j++)
                {
                    row[j] = values[j].Trim();
                }

                table.Rows.Add(row);

            }
        }
//save file - problem
        private void btnSave_Click(object sender, EventArgs e)
        {

            SaveFileDialog sfd = new SaveFileDialog(); //streamwriter
            sfd.Filter = "CSV Files (*.csv)|*.csv"; 
            if (sfd.ShowDialog() == DialogResult.OK)
            {
                using (StreamWriter writer = new StreamWriter(sfd.FileName))
                {
                    for (int i = 0; i < dataGridView1.Rows.Count - 0; i++)
                    {
                        for (int j = 0; j < dataGridView1.Columns.Count - 0; j++)
                        {

                            writer.Write("," + dataGridView1.Rows[i].Cells[j].Value.ToString());   //this line is broken =(
                        }
                        writer.WriteLine("");
                    }
                    writer.Close();
                }
            }
        }
    }

}