1.直接黏贴代码

public sealed class DaoHelper {
        private static DataTable _providers = null;
        private DbProviderFactory _pf = null;

        /// <summary>
        /// 静态构造函数，初始化数据提供程序列表。
        /// </summary>
        static DaoHelper() {
            _providers = DbProviderFactories.GetFactoryClasses();
        }
        /// <summary>
        /// 初始化 DAOHelper 类的新实例。
        /// </summary>
        public DaoHelper() {
        }
        /// <summary>
        /// 用指定的数据提供程序名称，初始化 DAOHelper 类的新实例。
        /// </summary>
        /// <param name="providerName">数据提供程序名称。</param>
        public DaoHelper(string providerName) {
            this.ProviderName = providerName;
        }

        private string m_providerName = "";

        /// <summary>
        /// 获取或设置数据提供程序名称。
        /// </summary>
        public string ProviderName {
            get { return m_providerName; }
            set {
                if(string.IsNullOrEmpty(value)){
                    throw new ArgumentNullException("value");
                }

                if(m_providerName != value) {
                    ChangeProvider(value);
                }
            }
        }

        /// <summary>
        /// 查找指定数名称的据提供程序行。
        /// </summary>
        /// <param name="providerInvariantName">数据提供程序名称。</param>
        /// <returns></returns>
        private DataRow FindProviderRow(string providerInvariantName) {
            if(_providers != null) {
                return _providers.Rows.Find(providerInvariantName);
            }
            throw new ProvidersNotFoundException();
        }

        /// <summary>
        /// 根据参数提供的数据提供程序名称，更改当前的数据提供程序工厂。
        /// </summary>
        /// <param name="providerName">数据提供程序名称。</param>
        private void ChangeProvider(string providerName) {
            if(string.IsNullOrEmpty(providerName)) {
                throw new ArgumentNullException("value");
            }

            DataRow providerRow = FindProviderRow(providerName);
            if(providerRow == null){
                throw new ProviderNotFoundException(providerName);              
            }
            _pf = DbProviderFactories.GetFactory(providerRow);
            m_providerName = providerName;
            
        }

        /// <summary>
        /// 创建数据源枚举器。
        /// </summary>
        /// <returns>返回创建成功的数据源枚举器。</returns>
        public DbDataSourceEnumerator CreateDataSourceEnumerator() {
            if(_pf == null) {
                throw new ProviderFactoryNullException();
            }
            return _pf.CreateDataSourceEnumerator();
        }

        /// <summary>
        /// 创建数据库连接。
        /// </summary>
        /// <returns>返回成功创建的连接对象。</returns>
        public DbConnection CreateConnection() {
            if(_pf == null) {
                throw new ProviderFactoryNullException();
            }
            return _pf.CreateConnection();
        }

        /// <summary>
        /// 创建数据库连接，并用指定的数据库连接字符串打开。
        /// </summary>
        /// <param name="connectionString">数据库连接字符串。</param>
        /// <returns>返回成功创建并打开的连接对象。</returns>
        public DbConnection CreateConnection(string connectionString){
            DbConnection result = CreateConnection();
            result.ConnectionString = connectionString;
            result.Open();
            return result;
        }

        /// <summary>
        /// 创建数据库连接字符串生成器。
        /// </summary>
        /// <returns>返回成功创建的连接对象。</returns>
        public DbConnectionStringBuilder CreateConnectionStringBuilder() {
            if(_pf == null) {
                throw new ProviderFactoryNullException();
            }
            return _pf.CreateConnectionStringBuilder();
        }

        /// <summary>
        /// 创建数据适配器。
        /// </summary>
        /// <returns></returns>
        public DataAdapter CreateAdapter() {
            if(_pf == null) {
                throw new ProviderFactoryNullException();
            }
            return _pf.CreateDataAdapter();
        }
    }