我用from1 作为登录窗口 ,点击登录后打开from2 这时我想关闭from1, 该在什么地方使用close
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 丽控业务管理系统
{
  public partial class frmlogin : Form
  {
  public static string struserid;
  public static string strusermc;
  public frmlogin()
  {
  InitializeComponent();
  }

  private void btnlogin_Click(object sender, EventArgs e)
  {
  string soure = "database=lkdata;server=127.0.0.1;user id=sa;password=";
  string sqlqxdz = "select qxdm from tb_qxdz where yhdm='" + txtbyhdm.Text.ToString().Trim() + "'";
  string sqlyh = "select * from tb_yh where yhdm='" + txtbyhdm.Text.ToString().Trim() + "' and yhkl='" + txtbyhkl.Text.ToString().Trim() + "'";
  SqlConnection mycon = new SqlConnection(soure);
  SqlDataAdapter da = new SqlDataAdapter(sqlyh, mycon);
  SqlDataAdapter da1 = new SqlDataAdapter(sqlqxdz, mycon);
  DataSet ds = new DataSet();
  da.Fill(ds, "tb_yh");
  da1.Fill(ds, "tb_qxdz");
  if (txtbyhdm.Text.ToString().Trim() == null)
  { MessageBox.Show("用户代码为空"); }
  if (txtbyhkl.Text.ToString().Trim() == null)
  { MessageBox.Show("用户口令为空"); }
   
  if (ds.Tables[0].Rows.Count >= 0)
  {
  struserid = ds.Tables[0].Rows[0]["yhdm"].ToString().Trim();
  strusermc = ds.Tables[0].Rows[0]["yhmc"].ToString().Trim();
  frmmain frm_main = new frmmain();
  frm_main.Show();

  for (int i = 0; i < ds.Tables[1].Rows.Count; i++)
  {
  switch (ds.Tables[1].Rows[i]["qxdm"].ToString().Trim())
  {
  case "01":
  frm_main.tlmxtgl.Enabled = true;
  frm_main.tlmxtgl.Visible = true;
  break;
  case "0101":
  frm_main.tlmyhgl.Enabled = true;
  frm_main.tlmyhgl.Visible = true;
  break;
  case "0102":
  frm_main.tlmqxgl.Enabled = true;
  frm_main.tlmqxgl.Visible = true;
  break;
  case "0103":
  frm_main.tlmxgkl.Enabled = true;
  frm_main.tlmxgkl.Visible = true;
  break;
  case "0104":
  frm_main.tlmtc.Enabled = true;
  frm_main.tlmtc.Visible = true;
  break;
  }

  }
  }
  else
  { Messa