用户管理：登录窗体通过ShowDialog()方法实现切换

        /// <summary>
        /// 成功登录后的主窗体MainForm.cs的OnShown方法
        /// </summary>
        /// <param name="e"></param>
        protected override void OnShown(EventArgs e)
        {
            base.OnShown(e);
            #region 登录框
            btnProject.Enabled = false;
            DialogResult result = DialogResult.No;
            LoginForm login = new LoginForm();
            result = login.ShowDialog();
            btnProject.Enabled = true;
            #endregion
            #region 权限设置
            if (!BaseClass.CheckRolePower(UserAccessLeve.TestProject))
                btnProject.Visibility = DevExpress.XtraBars.BarItemVisibility.Never;
            #endregion
            SetLoadString("Initing ...");//登录成功后，进入主界面过程中加载等待框
        }