C# - system.nullreferenceexception: “object reference not set to an instance of an object.” - saving as excel from datagridview from another panel

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Hello,

I'm trying to save data as excel from datagridview from another panel, but first I have to retrieve all from Mysql database.

private void btn_Retrieve_all_Click(object sender, EventArgs e)
    {
        string conn2 = "server=192.168.193.10;port=3306;Database=database;uid=root;pwd=123;CharSet=utf8mb4;";

        string query2 = "SELECT * FROM equipments";

        MySqlConnection connection2 = new MySqlConnection(conn2);
        MySqlDataAdapter adapter2 = new MySqlDataAdapter(query2, connection2);

        DataSet ds = new DataSet();
        adapter2.Fill(ds, "equipments");
        dataGridView1.DataSource = ds.Tables["equipments"];

        //dataGridView1.Columns[0].Visible = false;

        frm1.saveAsToolStripMenuItem.Enabled = true;
        frm1.excelToolStripMenuItem.Enabled = true;
        frm1.pDFToolStripMenuItem.Enabled = true;
        Delete_btn.Enabled = true;
        Save_btn.Enabled = true;
        Print_btn.Enabled = true;
    }

When i retrieve all I'd like to save this data to excel:

private void excelToolStripMenuItem_Click(object sender, EventArgs e)
        {
            saFDialog1.InitialDirectory = "C";
            saFDialog1.Title = "Save as Excel file";
            saFDialog1.FileName = "";
            saFDialog1.Filter = "Excel Files (2007)|*.xlsx|Excel Files(.CSV)|*.csv|Excel files (2003)|*.xls";

            if(saFDialog1.ShowDialog()!= DialogResult.Cancel)
            {
                Microsoft.Office.Interop.Excel.Application xcelApp = new Microsoft.Office.Interop.Excel.Application();
                xcelApp.Application.Workbooks.Add(Type.Missing);

                for (int i = 0; i < frm2.dataGridView1.Columns.Count + 1; i++)
                {
                    xcelApp.Cells[1, i] = frm2.dataGridView1.Columns[i - 1].HeaderText;
                }

                for (int i = 0; i < frm2.dataGridView1.Rows.Count; i++)
                {
                    for (int j = 0; j < frm2.dataGridView1.Columns.Count; j++)
                    {
                        xcelApp.Cells[i + 2, j + 1] = frm2.dataGridView1.Rows[i].Cells[j].Value.ToString();
                    }
                }

                xcelApp.ActiveWorkbook.SaveCopyAs(saFDialog1.FileName.ToString());
                xcelApp.ActiveWorkbook.Saved = true;
                xcelApp.Visible = true;
            }

However i added line code to declare opened form because datagridview is in another form (this case panel):

ITDeviceInfo frm2 = (ITDeviceInfo)Application.OpenForms["ITDeviceInfo"];

When I launched this application written in C# .NET and MySQL database I retrieved all then I chose [save] -> [Excel]. When i confirmed the place to save this excel file then appears this fail information:

Quote:
System.NullReferenceException: "Object reference not set to an instance of an object.

And indicates at code line:

for (int i = 0; i < frm2.dataGridView1.Columns.Count + 1; i++)

I have a question, what should I change at this code which saves the excel file from datagridview? Any Ideas? Thx for any help.

What I have tried:

In below are code sources:

Menu.cs

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 IT_equipment_program
{
    public partial class Menu : Form
    {
        public Menu()
        {
            InitializeComponent();
        }
        ITDeviceInfo frm2 = (ITDeviceInfo)Application.OpenForms["ITDeviceInfo"];

        private void iTDeviceInfoMNGToolStripMenuItem_Click(object sender, EventArgs e)
        {
            OpenForm<ITDeviceInfo>();
        }


        private void OpenForm<Myform>() where Myform : Form, new()
        {
            Form formular;
            formular = panelForm.Controls.OfType<Myform>().FirstOrDefault();

            if (formular == null)
            {
                formular = new Myform(); 
                formular.TopLevel = false;
                formular.FormBorderStyle = FormBorderStyle.None;
                formular.Dock = DockStyle.Fill;
                panelForm.Controls.Add(formular);
                panelForm.Tag = formular;
                formular.Show();
                formular.BringToFront();
            }

            else
            {
                formular.BringToFront();
            }
        }

        private void Menu_FormClosed(object sender, FormClosedEventArgs e)
        {
            Application.Exit();
        }

        private void excelToolStripMenuItem_Click(object sender, EventArgs e)
        {
            saFDialog1.InitialDirectory = "C";
            saFDialog1.Title = "Save as Excel file";
            saFDialog1.FileName = "";
            saFDialog1.Filter = "Excel Files (2007)|*.xlsx|Excel Files(.CSV)|*.csv|Excel files (2003)|*.xls";

            if(saFDialog1.ShowDialog()!= DialogResult.Cancel)
            {
                Microsoft.Office.Interop.Excel.Application xcelApp = new Microsoft.Office.Interop.Excel.Application();
                xcelApp.Application.Workbooks.Add(Type.Missing);

                for (int i = 0; i < frm2.dataGridView1.Columns.Count + 1; i++)
                {
                    xcelApp.Cells[1, i] = frm2.dataGridView1.Columns[i - 1].HeaderText;
                }

                for (int i = 0; i < frm2.dataGridView1.Rows.Count; i++)
                {
                    for (int j = 0; j < frm2.dataGridView1.Columns.Count; j++)
                    {
                        xcelApp.Cells[i + 2, j + 1] = frm2.dataGridView1.Rows[i].Cells[j].Value.ToString();
                    }
                }

                xcelApp.ActiveWorkbook.SaveCopyAs(saFDialog1.FileName.ToString());
                xcelApp.ActiveWorkbook.Saved = true;
                xcelApp.Visible = true;
            }
        }
    }
}

ITDeviceInfo.cs

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 DGVPrinterHelper;
using iTextSharp.text;
using iTextSharp.text.pdf;
using System.IO;
using Font = System.Drawing.Font;
using System.Text.RegularExpressions;
using System.Diagnostics;
using MySql.Data.MySqlClient;

namespace IT_equipment_program
{
    public partial class ITDeviceInfo : Form
    {
        MySqlConnection connection;
        MySqlCommand command;
        MySqlDataReader dr;
        public ITDeviceInfo()
        {
            InitializeComponent();
        }

        Menu frm1 = (Menu)Application.OpenForms["Menu"];

        private void Exit_btn_Click(object sender, EventArgs e)
        {
            this.Close();
            frm1.saveAsToolStripMenuItem.Enabled = false;
            frm1.excelToolStripMenuItem.Enabled = false;
            frm1.pDFToolStripMenuItem.Enabled = false;
        }

        private void btn_Retrieve_all_Click(object sender, EventArgs e)
        {
            string conn2 = "server=192.168.193.10;port=3306;Database=database;uid=root;pwd=123;CharSet=utf8mb4;";

            string query2 = "SELECT * FROM equipments";

            MySqlConnection connection2 = new MySqlConnection(conn2);
            MySqlDataAdapter adapter2 = new MySqlDataAdapter(query2, connection2);

            DataSet ds = new DataSet();
            adapter2.Fill(ds, "equipments");
            dataGridView1.DataSource = ds.Tables["equipments"];

            //dataGridView1.Columns[0].Visible = false;

            frm1.saveAsToolStripMenuItem.Enabled = true;
            frm1.excelToolStripMenuItem.Enabled = true;
            frm1.pDFToolStripMenuItem.Enabled = true;
            Delete_btn.Enabled = true;
            Save_btn.Enabled = true;
            Print_btn.Enabled = true;
        }      
    }
}

Posted 2-Mar-21 23:40pm

Member 10696161

Updated 3-Mar-21 0:33am

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

OriginalGriff · Answer 1 · 2021-03-03T00:33:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

C# - system.nullreferenceexception: “object reference not set to an instance of an object.” - saving as excel from datagridview from another panel

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C# - system.nullreferenceexception: “object reference not set to an instance of an object.” - saving as excel from datagridview from another panel

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