1     static class Program
 2     {
 3         /// <summary>
 4         /// 应用程序的主入口点。
 5         /// </summary>
 6         [STAThread]
 7         static void Main()
 8         {
 9             Application.EnableVisualStyles();
10             Application.SetCompatibleTextRenderingDefault(false);
11             Application.Run(new Form1());
12         }
13     }

        private void button1_Click(object sender, EventArgs e)
        {
            this.Hide();
             // here "this" is point to Form1,
             //so it means to hide the main Form  Form1
            new Form2().Show();                   
        }

1 Application.Run(new Form1());