C#

SimpleData wrote:

I've used Control.CheckForIllegalCrossThreadCalls = false when the form is
loaded

SimpleData wrote:

Control.CheckForIllegalCrossThreadCalls = false

using (FileStream fs = new FileStream(last_save_location, FileMode.Append, FileAccess.Write))
{
    fs.Write(message, 8, bytesRead - 8);
    fs.Close();
}

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

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form2 form2;
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (form2 == null)
            {
                form2 = new Form2(this);
                form2.Show();
            }
            else
            {
                form2.Show();
            }
            this.Hide();
            button2.BackColor = Color.Green;
        }
    }
}

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

namespace WindowsFormsApplication1
{
    public partial class Form2 : Form
    {
        public Form form1;
        public Form2(Form form1)
        {
            InitializeComponent();
            this.form1 = form1;
        }

        private void Form2_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (form1 == null)
            {
                form1 = new Form();
                form1.Show();
                
            }
            else
            {
                form1.Show();
            }
            this.Hide();
        }
    }
}

// Form1
public partial class Form1 : Form
{
    private Form2 form2;

    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        if (form2 == null)
        {
            // create instance and subscribe to events
            form2 = new Form2();
            form2.ButtonClicked += new EventHandler(form2_ButtonClicked);
            form2.FormClosing += new FormClosingEventHandler(form2_FormClosing);
        }
        form2.Show();
        Hide();
        button2.BackColor = Color.Green;
    }

    private void form2_ButtonClicked(object sender, EventArgs e)
    {
        Show();
        form2.Hide();
    }
    private void form2_FormClosing(object sender, FormClosingEventArgs e)
    {
        // unsubscribe and clear reference
        form2.ButtonClicked -= form2_ButtonClicked;
        form2.FormClosing -= form2_FormClosing;
        form2 = null;
        
        if (!Visible) // no forms visible so close to end application
            Close();
    }
}

// Form2
public partial class Form2 : Form
{
    public event EventHandler ButtonClicked;

    public Form2()
    {
        InitializeComponent();
    }

    protected virtual void OnButtonClicked(EventArgs e)
    {
        EventHandler eh = ButtonClicked;
        if (eh != null)
            eh(this, e);
    }

    private void button1_Click(object sender, EventArgs e)
    {
        OnButtonClicked(EventArgs.Empty);
    }
}