asp.net2.0编译错误

Question

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            string myconn = System.Configuration.ConfigurationManager.ConnectionStrings["connstr"].ConnectionString;          
            SqlConnection cn0 = new SqlConnection(myconn);
            cn0.Open();
            string mysql = "select * from Users";
            SqlCommand cm = new SqlCommand(mysql, cn0);
            SqlDataReader dr = cm.ExecuteReader();
            cn0.Close();
        }
编译的时候发生错误：
//string myconn = System.Configuration.ConfigurationManager.ConnectionStrings["connstr"].ConnectionString;// +$exception {"未将对象引用设置到对象的实例。"} System.Exception {System.NullReferenceException}

Answer 1

察看System.Configuration.ConfigurationManager.ConnectionStrings["connstr"]System.Configuration.ConfigurationManager.ConnectionStrings["connstr"].ConnectionString是否为　ｎｕｌｌ

Answer 2

改成:string myconn = System.Configuration.ConfigurationManager.ConnectionStrings["connstr"].ToString