ConnectionString 属性尚未初始化问题

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Configuration;
using System.Data.SqlClient;
using System.Data.Common;
namespace Application
{
    class Program
    {
        static void Main(string[] args)
        {
            var name = Console.ReadLine(); 
            var name2 = string.Empty;
            //Console.WriteLine("Hello {0} and {1} and ",name2,name);
            Console.WriteLine("Hello"+"  "+name);
            SqlConnection con = new SqlConnection();
            con.ConnectionString = System.Configuration.ConfigurationManager.AppSettings.Get("SchoolConnectionString");
            string sqlstr = "select * from bfEMP";
            SqlCommand cmd = new SqlCommand(sqlstr, con);
            cmd.Connection = con;
            con.Open();
            var Num =cmd.ExecuteNonQuery();
            con.Close();
           // Console.WriteLine(Num); 
        }
    }
}

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这个ConnectionString 字符串明明写了 怎么调试的时候没数据呀？求高手，本人菜鸟一枚，请详细帮忙解释一下