FormSwitch.cs

using System;
using System.Windows.Forms;
using Project1; 

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

        private void btnProject1_Click(object sender, EventArgs e)
        {
            Form1 f = new Form1();
            f.Show();
            this.Hide(); 
        }
    }
}