“DAL.DBHelper”的类型初始值设定项引发异常。
(配置文件)

<connectionStrings>
<add name="Conn" connectionString="Data Source=192.168.1.109;Initial Catalog=GuoHong;Persist Security Info=True;User ID=sa password=123"/>
</connectionStrings>

--------------------------------------------------
—BHelper）

    public class DBHelper
    {
        public static string Conn = ConfigurationManager.ConnectionStrings["ConnStr"].ConnectionString;

        public static DataTable ExecuteDataTable(string sql,params SqlParameter[] para)
        {
            using(SqlConnection conn=new SqlConnection(Conn))
            {
                conn.Open();
                using(SqlCommand cmd=conn.CreateCommand())
                {
                    cmd.CommandText = sql;
                    cmd.Parameters.AddRange(para);
                    SqlDataAdapter ada = new SqlDataAdapter(cmd);
                    DataSet ds = new DataSet();
                    ada.Fill(ds);
                    return ds.Tables[0];
                }
            }
        }        
    }

-----------------------------------
(Default.aspx.cs)

    protected void Page_Load(object sender, EventArgs e) 
 
    {
        string str = ConfigurationManager.ConnectionStrings["Conn"].ConnectionString.ToString();
        DataTable table = DBHelper.ExecuteDataTable("select * from customer" );
        GridView1.DataSource= table.DataSet;
        GridView1.DataBind();
    }

[解决办法]
ConfigurationManager.ConnectionStrings["ConnStr"]

<connectionStrings>
<add name="Conn" connectionString="Data Source=192.168.1.109;Initial Catalog=GuoHong;Persist Security Info=True;User ID=sa password=123"/>
</connectionStrings>

看出问题了吗？
[解决办法]
SqlDataAdapter 不需要手工打开数据库连接。