新手求注释，要详细，本人初学者

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace jinxiaocun
{
    public partial class frmWindows_login : Form
    {
        int nCount = 0;
        public static string strLogin = "";
        public static string strPwd = ""; 
        public static int nXuanzhong = 0;
        string strSQL;
        public static int nsystem;
        public static int ncustormer;
        public static int nsupplie;
        public static int nstockmanage;
        public static int nsellmanage;
        public static int nreservemanage;
        public frmWindows_login()
        {
            InitializeComponent();
        }

        private void frmWindows_login_Load(object sender, EventArgs e)
        {

        }

        private void btnDenglu_Click(object sender, EventArgs e)
        {
            strLogin = txtLogin.Text;//获取用户名的文本信息
            strPwd = txtPwd.Text;

            //用户名和密码不能为空
            if (strLogin == "" || strPwd == "") //||表示或者
            {
                MessageBox.Show("用户名和密码不能为空");
                return;//程序不再往后执行
            }
            if (strLogin.IndexOf("'") >= 0)
            {
                MessageBox.Show("密码中不能含有'");
                return;
            }
            
            if (rdbgly.Checked == true) { 
                string strCountsql = "select count(*) from logintable where login='" + strLogin + "'";
                int nCountresult = Convert.ToInt32(ConglyClass.returnStringSql(strCountsql));
                if (nCountresult != 1){MessageBox.Show("你不是管理员/员工，无法登录！"); return;}
                nXuanzhong=1;
            }
            else if (rdbkehu.Checked == true)
            {
                 string strCountsql = "select count(*) from custormer where Clogin='" + strLogin + "'";
                 int nCountresult = Convert.ToInt32(ConglyClass.returnStringSql(strCountsql));
                 if (nCountresult != 1){MessageBox.Show("你不是客户，无法登录！"); return;}
                 nXuanzhong=2;
            }
            else { MessageBox.Show("请选择身份，再登录！"); return;}
            if(nXuanzhong==1){
            strSQL = "select password from logintable where login='" + strLogin + "'";
            }else{
            strSQL = "select CPassword from custormer where CLogin='" + strLogin + "'";
            }
            string strPwd2 = ConglyClass.returnStringSql(strSQL);
            if (strPwd == strPwd2)//判断密码是对的
            {
                this.Hide();
               if (nXuanzhong == 1) {
                   string strsqlcount = "select COUNT(*) from limits_authority where [login]='" + strLogin + "'";
                   if (Convert.ToInt32(ConglyClass.returnStringSql(strsqlcount)) == 1)
                   {
                       string strsql = "SELECT system,custormer,supplie,stockmanage,sellmanage,reservemanage FROM limits_authority where [login]='"+strLogin+"'";
                       SqlDataReader sdrread = ConglyClass.getSqlDataReader(strsql);
                       sdrread.Read();
                       nsystem = Convert.ToInt32(sdrread["system"]);
                       ncustormer = Convert.ToInt32(sdrread["custormer"]);
                       nsupplie=Convert.ToInt32(sdrread["supplie"]);
                       nstockmanage=Convert.ToInt32(sdrread["stockmanage"]);
                       nsellmanage=Convert.ToInt32(sdrread["sellmanage"]);
                       nreservemanage=Convert.ToInt32(sdrread["reservemanage"]);
                   }
                   else {
                       nsystem = 0;
                       ncustormer = 0;
                       nsupplie = 0;
                       nstockmanage = 0;
                       nsellmanage = 0;
                       nreservemanage = 0;
                   }
                   frmSystem_interface frm = new frmSystem_interface();
                   frm.ShowDialog();
               }
              
              if (frmChange_Password.nUpdate == 2) { this.Show(); txtPwd.Text = ""; frmChange_Password.nUpdate = 1; }  else { this.Close(); } 
            }
            else
            {
                //增加第一个功能：错误超过三次，则系统关闭，提示使用变量nCount
                nCount++;//值自动加1
                MessageBox.Show("密码输入错误！", "ERROR", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                if (nCount >= 3) this.Close();
            }     
        }

        private void btnRegister_Click(object sender, EventArgs e)
        {
            frmWindows_register frm2 = new frmWindows_register();
            frm2.ShowDialog();
            if (frmWindows_register.nRegister_status == 1) { 
            txtLogin.Text = frmWindows_register.strLogin;
            txtPwd.Text = "";}
        }

        private void rdbkehu_CheckedChanged(object sender, EventArgs e)
        {

        }

        private void rdbgly_CheckedChanged(object sender, EventArgs e)
        {

        }
    }
}//员工基本信息维护和客户资料管理