using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    static class Program
    {
        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Login());
        }
    }
}

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Login : Form
    {
        public Login()
        {
            InitializeComponent();
        }

        /// <summary>
        /// 登录
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void button1_Click(object sender, EventArgs e)
        {
            //登录成功

            (new Main()).Show();//显示主窗体
            this.Visible = false;//隐藏登录窗体,不能关闭
        }
    }
}

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Main : Form
    {
        public Main()
        {
            InitializeComponent();
        }

        private void Main_FormClosed(object sender, FormClosedEventArgs e)
        {
            Application.Exit();//关闭所有应用程序窗体
        }
    }
}