c# ConfigurationManager.AppSettings取不到值？？ 命名空间加了using System.Configuration;，引用也加了，config文件也没碰

Question

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

namespace WindowsFormsApplication1
{
public partial class Frm_1 : Form
{
public Frm_1()
{
InitializeComponent();
string str;
str = ConfigurationManager.AppSettings[""];
if (str == null)
MessageBox.Show("str为空");
this.Text = str;

    }

    private void button1_Click(object sender, EventArgs e)
    {
        string str1;
        str1 = ConfigurationManager.AppSettings["User"];
        MessageBox.Show( str1);
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        MessageBox.Show("窗体正在加载");
    }

    private void Frm_1_FormClosing(object sender, FormClosingEventArgs e)
    {
        MessageBox.Show("正在关闭窗口，是否退出？");

    }
}

}

Answer 1

str = ConfigurationManager.AppSettings[""];
这里写的是哪个键？

在你的 应用程序名.config （不是应用程序.vshost.config） 里面打开，内容检查下，配置项有没有