System.NullReferenceException: 未将对象引用设置到对象的实例问题~

{
    /// <summary>
    /// 类，用于数据访问的类。
    /// </summary>
    public class Database : IDisposable
    {
        /// <summary>
        /// 保护变量，数据库连接。
        /// </summary>
        protected SqlConnection Connection;

        /// <summary>
        /// 保护变量，数据库连接串。
        /// </summary>
        protected String ConnectionString;
        
        /// <summary>
        // 构造函数。
        /// </summary>
        /// <param name="DatabaseConnectionString">数据库连接串</param>
        public Database()
        {
            ConnectionString = ConfigurationManager.ConnectionStrings["QuestionManagerConnectionString"].ToString();
        }

        /// <summary>
        /// 析构函数，释放非托管资源
        /// </summary>
        ~Database()
        {
            try
            {
                if (Connection != null)
                    Connection.Close();
            }
            catch{}
            try
            {
                Dispose();
            }
            catch{}
        }

        /// <summary>
        /// 保护方法，打开数据库连接。
        /// </summary>
        protected void Open()
        {
            if (Connection == null)
            {
                Connection = new SqlConnection(ConnectionString);
            }
            if (Connection.State.Equals(ConnectionState.Closed))
            {
                try
                {
                    Connection.Open();
                }
                catch(SqlException e)
                {

                    //if (!File.Exists("error.txt"))
                    //{
                    //    File.Create("error.txt");
                    //}
                    //FileStream fs = new FileStream("error.txt", FileAccess.ReadWrite);
                    //StreamWriter w = new StreamWriter(fs);
                    //fs.Write(e.Message.
                  
                  
                    //string strMessage = e.Message;
                    ////以下把信息写入windows日志
                    ////要把aspnet用户添加到管理员组中，以便有写注册表权限
                    //if (!EventLog.SourceExists("TQMS"))
                    //    EventLog.CreateEventSource("TQMS", "Application");
                    //    //EventLog.CreateEventSource("mySource", "myLog");
                    //EventLog Event = new EventLog();
                    //Event.Source = "TQMS";
                    //Event.WriteEntry(strMessage, EventLogEntryType.Warning);
                    ////EventLog.Delete("myLog");
                  
                    //throw new Exception("我处理不了，请最高人民法院处理！");
                }
            }
        }

        /// <summary>
        /// 公有方法，关闭数据库连接。
        /// </summary>
        public void Close()
        {
            if (Connection != null)
                Connection.Close();
        }

        /// <summary>
        /// 公有方法，释放资源。
        /// </summary>
        public void Dispose()
        {
            // 确保连接被关闭
            if (Connection != null)
            {
                Connection.Dispose();
                Connection = null;
            }                
        }

        /// <summary>
        /// 公有方法，获取数据，返回一个SqlDataReader （调用后注意调用SqlDataReader.Close()）。
        /// </summary>
        /// <param name="SqlString">Sql语句</param>
        /// <returns>SqlDataReader</returns>
        public SqlDataReader GetDataReader(String SqlString)
        {
            Open();
            SqlCommand cmd = new SqlCommand(SqlString,Connection);
            return cmd.ExecuteReader();
        }

        /// <summary>
        /// 公有方法，获取数据，返回一个SqlDataReader （调用后注意调用SqlDataReader.Close()、Database.Close()）。
        /// </summary>
        /// <param name="SqlString">Sql语句</param>
        /// <returns>SqlDataReader</returns>
        public SqlDataReader GetSaDataReader(String SqlString)
        {
            Open();
            SqlCommand cmd = new SqlCommand(SqlString,Connection);
            return cmd.ExecuteReader();
        }

        /// <summary>
        /// 公有方法，获取数据，返回一个DataSet。
        /// </summary>
        /// <param name="SqlString">Sql语句</param>
        /// <returns>DataSet</returns>
        public DataSet GetDataSet(String SqlString)
        {
            Open();
            SqlDataAdapter adapter = new SqlDataAdapter(SqlString,Connection);
            DataSet dataset = new DataSet();
            adapter.Fill(dataset);
            Close();
            return dataset;
        }

        /// <summary>
        /// 公有方法，获取数据，返回一个DataTable。
        /// </summary>
        /// <param name="SqlString">Sql语句</param>
        /// <returns>DataTable</returns>
        public DataTable GetDataTable(String SqlString)
        {
            DataSet dataset = GetDataSet(SqlString);
            dataset.CaseSensitive = false;
            return dataset.Tables[0];
        }

        /// <summary>
        /// 公有方法，获取数据，返回一个DataRow。
        /// </summary>
        /// <param name="SqlString">Sql语句</param>
        /// <returns>DataRow</returns>
        public DataRow GetDataRow(String SqlString)
        {
            DataSet dataset = GetDataSet(SqlString);
            
            dataset.CaseSensitive = false;
            if (dataset.Tables[0].Rows.Count>0)
            {
                return dataset.Tables[0].Rows[0];
            }
            else
            {
                return null;
            }
        }

        /// <summary>
        /// 公有方法，执行Sql语句。
        /// </summary>
        /// <param name="SqlString">Sql语句</param>
        /// <returns>对Update、Insert、Delete为影响到的行数，其他情况为-1</returns>
        public int ExecuteSQL(String SqlString)
        {
            int count = -1;
            Open();
            try
            {
                SqlCommand cmd = new SqlCommand(SqlString,Connection);
                count = cmd.ExecuteNonQuery();
            }
            catch
            {
                count = -1;
            }
            finally
            {
                Close();
            }
            return count;
        }

        /// <summary>
        /// 公有方法，执行一组Sql语句。
        /// </summary>
        /// <param name="SqlStrings">Sql语句组</param>
        /// <returns>是否成功</returns>
        public bool ExecuteSQL(String[] SqlStrings)
        {
            bool success = true;
            Open();
            SqlCommand cmd = new SqlCommand();
            SqlTransaction trans = Connection.BeginTransaction();
            cmd.Connection = Connection;
            cmd.Transaction = trans;
            try
            {
                foreach (String str in SqlStrings)
                {
                     = str;
                    cmd.ExecuteNonQuery();
                }
                ();
            }
            catch
            {
                success = false;
                trans.Rollback();
            }
            finally
            {
                Close();
            }
            return success;
        }

        /// <summary>
        /// 公有方法，执行一组Sql语句。
        /// </summary>
        /// <param name="SqlStrings">Sql语句组</param>
        /// <returns>是否成功</returns>
        public bool ExecuteSQL(ArrayList SqlStrings)
        {
            bool success = true;
            Open();
            SqlCommand cmd = new SqlCommand();
            SqlTransaction trans = Connection.BeginTransaction();
            cmd.Connection = Connection;
            cmd.Transaction = trans;
            try
            {
                foreach (String str in SqlStrings)
                {
                     = str;
                    cmd.ExecuteNonQuery();
                }
                ();
            }
            catch
            {
                success = false;
                trans.Rollback();
            }
            finally
            {
                Close();
            }
            return success;
        }

        /// <summary>
        /// 公有方法，在一个数据表中插入一条记录。
        /// </summary>
        /// <param name="TableName">表名</param>
        /// <param name="Cols">哈西表，键值为字段名，值为字段值</param>
        /// <returns>是否成功</returns>
        public bool Insert(String TableName,Hashtable Cols)
        {
            int Count = 0;

            if (Cols.Count<=0)            
            {
                return true;
            }

            String Fields = " (";
            String Values = " Values(";            
            foreach(DictionaryEntry item in Cols)
            {
                if (Count!=0)
                {
                    Fields += ",";
                    Values += ",";
                }
                Fields += "["+item.Key.ToString()+"]";
                Values += item.Value.ToString();
                Count ++;
            }
            Fields += ")";
            Values += ")";

            String SqlString = "Insert into "+TableName+Fields+Values;

            String[] Sqls = {SqlString};
            return ExecuteSQL(Sqls);
        }

    
        /// <summary>
        /// 公有方法，更新一个数据表。
        /// </summary>
        /// <param name="TableName">表名</param>
        /// <param name="Cols">哈西表，键值为字段名，值为字段值</param>
        /// <param name="Where">Where子句</param>
        /// <returns>是否成功</returns>
        public bool Update(String TableName,Hashtable Cols,String Where)
        {
            int Count = 0;
            if (Cols.Count<=0)            
            {
                return true;
            }
            String Fields = " ";
            foreach(DictionaryEntry item in Cols)
            {
                if (Count!=0)
                {
                    Fields += ",";
                }
                Fields += "["+item.Key.ToString()+"]";
                Fields += "=";
                Fields += item.Value.ToString();
                Count ++;
            }
            Fields += " ";

            String SqlString = "Update "+TableName+" Set "+Fields+Where;

            String[] Sqls = {SqlString};
            return ExecuteSQL(Sqls);
        }        
    }
}


源码。。。