C# 中的 ConfigurationManager类引用方法

c#添加了Configuration;后，竟然找不到 ConfigurationManager 这个类，后来才发现：虽然引用了using System.Configuration;这个包，但是还是不行的。

实例：DataGridView显示数据

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using DevExpress.XtraEditors;
using System.Data.SqlClient;
using System.Configuration;

namespace WindowsApplication1
{
public partial class XtraForm1 : DevExpress.XtraEditors.XtraForm
{
public XtraForm1()
{
InitializeComponent();
}

private void button1_Click(object sender, EventArgs e)
{
string customers = "select * from Customers";
using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["northwind"].ConnectionString))
{

DataSet dt = new DataSet();
SqlDataAdapter da = new SqlDataAdapter(customers, con);
da.Fill(dt, "Customers");
dataGridView1.AutoGenerateColumns = true;
dataGridView1.DataSource = dt;
dataGridView1.DataMember = "Customers";
}
}
}
}