using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            Form2 f2 = new Form2(this);
            f2.Show();
            this.Hide();
        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
    public partial class Form2 : Form
    {
        public Form1 prof1;
        public Form2(Form1 f1)
        {
            InitializeComponent();
            prof1 = f1;
        }
        private void button1_Click(object sender, EventArgs e)
        {
            prof1.Show();
            this.Close();
        }
    }
}