C# form switch and vice versa

Question

Assume that I have a C# Solution with 3 projects Main, Program1, Program2.
I want to have a "Main form", when I click on button "Program1" the main form will be hidden, Program1 will be showed, and when I close Program1, the Main form will return.
How can I do this? I tried add Program1 and PRogram2 as Reference to Project Main and code like below in Main, it works for call Program1, but can't handle event Program1.closed() because when I try to reference Main to Program1, it error

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Microsoft Visual Studio
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A reference to 'Main' could not be added. Adding this project as a reference would cause a circular dependency.
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OK   
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I searched Google and got nothing helpful!

using System;
using System.Windows.Forms;

namespace Switch
{
    public partial class Main : Form
    {
        public Main()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Program1.Form1 pf1 = new Program1.Form1();
            pf1.Show();
            this.Hide(); 
        }
    }
}

Answer 1

Thanks everyone for answers!
After confirmed with customer, they don't strictly need the "mainform" to be hidden, so I came with another easier solution:
1. For the "child form", I use ShowDiaglog() instead of Show()

    private void btnChildForm1_Click(object sender, EventArgs e)
    {
        var frm = new ChildForm1();
        frm.ShowDialog();
    }

2. For the mainform, I use mutex to force it to be only 1 instance:

   static class Program
   {
       /// <summary>
       /// The main entry point for the application.
       /// </summary>
       /// 
   
   
       [STAThread]
       static void Main()
       {
           var mutex = new Mutex(true, "MainForm", out var result);
           if (!result)
           {
               MessageBox.Show("Running!");
               return;
           }
           Application.EnableVisualStyles();
           Application.SetCompatibleTextRenderingDefault(false);
           Application.Run(new MainForm());
           GC.KeepAlive(mutex);
       }
   }