请教一个数据库连接类的调用方法
using System;
using System.Collections;
using System.Collections.Specialized;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
namespace myDbMang
{
    /// <summary>
    ///
    /// </summary>
    public abstract class DbManagerSQL
    {
        //数据库连接字符串(web.config来配置)
        //<add key="ConnectionString" value="server=127.0.0.1;database=DATABASE;uid=sa;pwd=" />  
        protected static string connectionString = ConfigurationSettings.AppSettings["ConnectionString"];
        public DbManagerSQL()
        {
        }

        /// <summary>
        /// 执行查询语句，返回DataSet
        /// </summary>
        /// <param >查询语句</param>
        /// <returns>DataSet</returns>
        public static DataSet Query(string SQLString)
        {
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                DataSet ds = new DataSet();
                try
                {
                    ds.Clear();
                    connection.Open(); // 出错位置：提示 ConnectionString 属性尚未初始化。
                    SqlDataAdapter command = new SqlDataAdapter(SQLString, connection);
                    command.Fill(ds, "ds");
                }
                catch (System.Data.SqlClient.SqlException ex)
                {
                    throw new Exception(ex.Message);
                }
                return ds;
     &nb