http://www.cnblogs.com/leotian/archive/2008/12/02/1346004.html

 protected void Page_Load(object sender, EventArgs e)
        {
            CheckLogin MyChk = new CheckLogin();
            title.InnerHtml = "<b>数据库连接配置</b>";


            if (!IsPostBack)
            {
                Configuration config =System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration(HttpContext.Current.Request.ApplicationPath);
                string myConnect =config.ConnectionStrings.ConnectionStrings["MyApp.Properties.Settings.SqlConnectSetting"].ToString();

                string[] arycon = myConnect.Split(';');
                int i, li_index;
                for (i = 0; i < arycon.Length; i++)
                {
                    if (arycon[i].IndexOf("Data Source") > -1)
                    {
                        li_index = arycon[i].IndexOf("=");
                        TextBox1.Text = arycon[i].Substring(li_index + 1);
                    }
                    if (arycon[i].IndexOf("Initial Catalog") > -1)
                    {
                        li_index = arycon[i].IndexOf("=");
                        TextBox2.Text = arycon[i].Substring(li_index + 1);
                    }
                    if (arycon[i].IndexOf("User ID") > -1)
                    {
                        li_index = arycon[i].IndexOf("=");
                        TextBox3.Text = arycon[i].Substring(li_index + 1);
                    }
                    if (arycon[i].IndexOf("Password") > -1)
                    {
                        li_index = arycon[i].IndexOf("=");
                        TextBox4.Text = arycon[i].Substring(li_index + 1);
                    }
                }
            }
        }

;

            ConnectionStringsSection csSection = config.ConnectionStrings;
            csSection.ConnectionStrings[csName].ConnectionString = ls_newstring;

            config.Save(ConfigurationSaveMode.Modified);

            //IIS将重新启动
        }
    }