C#

private void button1_Click(object sender, EventArgs e)
        {
             Form2 mw2 = new Form2();
             mw2.Show();
        }

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

namespace Modales_window
{
    public partial class Form1 : Form
    {
        Form2 mw2 = new Form2();

        // Form1 form = (Form1)Application.OpenForms[0];
        // Form2 form2 = (Form2)Application.OpenForms[1];

        public Form1()
        {
            mw2.closingeventhandler += new Form2.ClosingEventHandler(mw2_closingeventhandler);
            InitializeComponent();
        }
        void mw2_closingeventhandler()
        {
            //mw2.Dispose();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            // Cannot access a disposed object.
            // Object name: 'Form2'.
            if (!mw2.Created)
            {
                mw2.Show();
            }
            else
            {
                MessageBox.Show("already open", "", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            textBox1.Text = Application.OpenForms.Count.ToString();
        }
}