首先：窗体的两个属性 AcceptButton ，和 CancelButton 分别设置为窗体上“确定”和“取消”按钮的名字：bnok 和 bncancel.
其次：确定按钮和取消按钮的"DialogResult"属性分别设置为：ok , cancel
然后：窗体上的控件textbox要在类外部读出来那么设置其 modifier属性为public即可
接着：

            int errCount = 0;
            while (errCount < 3)
            {
                FrmLogin flog = new frmLogin();
                flog.ShowDialog(this);
                if (flog.DialogResult == DialogResult.OK)
                {
                    flog.Username = flog.txtusername.Text;
                    flog.Password = flog.txtpassword.Text;
                    if (flog.Username == "liumu" &&
                       flog.Password == "198331622")
                    {
                        MessageBox.Show("欢迎访问该系统", "登录成功");

                        ShowWelcomePage();
                        break;
                    }
                    else
                    {
                        errCount++;
                        MessageBox.Show("用户名或密码有误请核对后重新输入", "错误提示");
                    }
                }
                else if (flog.ShowDialog() == DialogResult.Cancel)
                {
                    Application.Exit();
                }
            }

            if (errCount == 3)
            {
                MessageBox.Show("您三次输入有误,被迫推出系统，Sorry!", "退出提示");
                Application.Exit();
            }