当不在独立 exe 内部运行时，必须指定 exePath。怎么回事呢？？

我按书上的代码做连接字符串的加密：代码如下：using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
           
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        String strconnectionname = "Local SQLEXPRESS";
        ConnectionStringSettings setting;
        setting =ConfigurationManager.ConnectionStrings[strconnectionname];
        if (setting == null)
        {
            setting = new ConnectionStringSettings();
            setting.Name = strconnectionname;
            setting.ConnectionString = @"Data Source=.\SQLExpress;" + "AttachDbFilename=D:\\asp\\yingchangstring\\App_Data\\user.mdf;"+"Integrated  Security=True;";
            Configuration config;
            config=ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);   //这句运行到这就会出现错误： 当不在独立 exe 内部运行时，必须指定 exePath
            config.ConnectionStrings.ConnectionStrings.Add(setting);
            config.Save();
            config.ConnectionStrings.SectionInformation.ProtectSection(null);  //这句给连接字符串加密
        }else
        {
            using (SqlConnection cn = new SqlConnection(setting.ConnectionString))
            {
                try
                {
                    SqlCommand da = new SqlCommand("select count(*) from password where  usename='" + DropDownList1.Text + "' and mima='" + TextBox1.Text + "'", cn);
                    cn.Open();
                    int i = (int)da.ExecuteScalar();
                    if (i == 1)
                    {
                        Response.Redirect("Default2.aspx");
                    }                }
                catch
                {
                }
            }
        }
    }
}
  config=ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);   //这句运行到这就会出现错误： 当不在独立 exe 内部运行时，必须指定 exePath 
请问哪里错了。该怎么改呢。