C# winform主界面打开并关闭登录界面的方法

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

/// <summary>
/// login
/// </summary>
/// <param name="sender"></param>
 /// <param name="e"></param>
    private void btnLogin_Click(  sender, EventArgs e)
    {
      if (txtPassword.Text == "12345678")
      {
        UI ui = new UI();
        this.Visible = false;
        ui.ShowDialog();//此处不可用Show()
        this.Dispose();
        this.Close();
      }
      else
      {
        MessageBox.Show("Password is incorrect ", "Prompt message",MessageBoxButtons.OK,MessageBoxIcon.Error);
      }
    }

    /// <summary>
    /// exit
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void btnExit_Click(  sender, EventArgs e)
    {
      this.Dispose();
      this.Close();
}

/// <summary>
/// 应用程序的主入口点。
/// </summary>
    [STAThread]
    static void Main()
    {
      Application.EnableVisualStyles();
      Application.SetCompatibleTextRenderingDefault(false);
      Login login = new Login();
      login.ShowDialog();
      if (login.DialogResult == DialogResult.OK)
      {
        login.Dispose();
        Application.Run(new UI());
      }
      else if (login.DialogResult == DialogResult.Cancel)
      {
        login.Dispose();
        return; 
      }
}

/// <summary>
/// login
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
    private void btnLogin_Click(  sender, EventArgs e)
    {
      if (txtPassword.Text == "12345678")
      {
        this.DialogResult = DialogResult.OK;
        this.Dispose();
        this.Close();
      }
      else
      {
        MessageBox.Show("Password is incorrect ", "Prompt message",MessageBoxButtons.OK,MessageBoxIcon.Error);
      }
    }

    /// <summary>
    /// exit
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void btnExit_Click(  sender, EventArgs e)
    {
      this.DialogResult = DialogResult.Cancel;
      this.Dispose();
      this.Close();
}