问ConfigurationManager不可用

EN

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.SqlClient;
using System.Configuration;

namespace Gmos.Halbtax.Models
{
    static class Program
    {
        public static void connectDB()
        {
            string connString = ConfigurationManager.AppSettings["ConnString"].ToString();
            SqlConnection conn = new SqlConnection(connString);          
        }
    }
}