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 Workshop2
{
    public partial class Form1 : Form
    {
        //Form2 f2 = new Form2();
        public Form2 f2;
        public Form1()
        {
            InitializeComponent();
        }
        private void showForm2button_Click(object sender, EventArgs e)
        { 
            if (f2 == null)
            {
                f2 = new Form2();// you need to set the textbox in form2 to public
                f2.textBox1.Text = "Initially Set from form1";// need to ensure if the form is closed it is dereferenced
                f2.FormClosed += delegate { 
                    f2 = null; 
                };
            }
            f2.Show();
        }
    }
}