基于接口编程。

举一个常用的例子，比如程序的数据库访问层能够同时支持,SQLServer, Oracle等多种数据库。这样，在实例化ADO.net对象和具体操作时候，就要走不同的分支。一个是SQL Server的，一个是Oracle的。 这时候，就需要不同的条件（可以是后台设置的使用SQL or Oracle or ...），实例化不同数据库访问类的对象。

 //接口默认public,加上public修饰符也不报错
    interface IUserService
    {
        //接口方法不能有访问修饰符,否则报错
        string GetUserName();
        bool IsExists();
    }

 

public class SQLUserService:IUserService
    {
        //实现接口的方法必须是 public 的
        public string GetUserName()
        {
            return "SQLServer return username: SQL";
        }

        public bool IsExists()
        {
            return true;
        }
    }

 public class OracleUserService : IUserService
    {
        //实现接口的方法必须是 public 的
        public string GetUserName()
        {
            return "Oracle return username: Oracle";
        }

        public bool IsExists()
        {
            return true;
        }
    }

 IUserService userBLL = null;
            //根据不同条件，给接口实现不同的类的实例。
            if (Request.Form["DB"] != null && Request.Form["DB"].ToString() == "SQL")
            {
                userBLL = new SQLUserService();
            }
            else
            {
                userBLL = new OracleUserService();
            }
            string name = userBLL.GetUserName();

 

我们Service端的连接字符串保存到Application对象中，但是Test项目是一个类库项目，不是Web Application,获取不到System.Web.HttpContext.Current下的东西，这时候，解决方案是扩展一下获取连接字符串的方法，WEB application还是从application对象取，但是如果是类库项目，从System.Collections.Hashtable中设值和取值。

IConnectionStringStorageContainer接口：

 public interface IConnectionStringStorageContainer
    {
        string GetConnectionString(string key);

        void Store(string key, string connectionString);

        void Clear();
    }

包含3个方法。获取连接字符串，设值连接字符串，清除。要实现的是，根据不同环境， 从application/Hashtable 对象 获取/设值连接字符串

具体实现类HashtableConnectionStringStorageContainer，用于类库项目：

public class HashtableConnectionStringStorageContainer: IConnectionStringStorageContainer
    {
        private static readonly Hashtable _connectionStrings = new Hashtable();

        public string GetConnectionString(string key)
        {
            string connectionString = string.Empty;

            if (_connectionStrings.Contains(key))
                connectionString = _connectionStrings[key].ToString();

            return connectionString;
        }

        public void Store(string key, string connectionString)
        {
            if (_connectionStrings.Contains(key))
                _connectionStrings[key] = connectionString;
            else
                _connectionStrings.Add(key, connectionString); 

        }

        public void Clear()
        {
            _connectionStrings.Clear();
        }
    }

具体实现类HttpConnectionStringStorageContainer，用于Web Application:

public class HttpConnectionStringStorageContainer : IConnectionStringStorageContainer
    {
        public string GetConnectionString(string key)
        {
            string connectionString = string.Empty;
            if (System.Web.HttpContext.Current.Application.AllKeys.Contains(key))
                connectionString = System.Web.HttpContext.Current.Application[key].ToString();
            return connectionString;
        }

        public void Store(string key, string connectionString)
        {
            if (System.Web.HttpContext.Current.Application.AllKeys.Contains(key))
                System.Web.HttpContext.Current.Application[key] = connectionString;
            else
                System.Web.HttpContext.Current.Application.Add(key, connectionString);
        }

        public void Clear()
        {
            throw new NotImplementedException();
        }
    }

调用的地方，根据条件不同，给接口实例化不同的类的对象，这里写在一个单独的类中：

 public class ConnectionStringStorageFactory
    {
        public static IConnectionStringStorageContainer _connectionStringStorageContainer;

        public static IConnectionStringStorageContainer CreateStorageContainer()
        {
            if (_connectionStringStorageContainer == null)
            {
                if (System.Web.HttpContext.Current == null)
                    _connectionStringStorageContainer = new HashtableConnectionStringStorageContainer();
                else
                    _connectionStringStorageContainer = new HttpConnectionStringStorageContainer();
            }

            return _connectionStringStorageContainer;
        }
    }

这样，当WEB项目调用的时候，实例化的是HttpConnectionStringStorageContainer类，

DAL中实例化EF DBContext数据库上下文对象的时候，根据不同ConnectionStringStorageContainer类:

public string GetWellMeOwnedFlagByGMPI(int? gmpi_id)
        {
            List byteFlag = new List { 1 };

            using (var edw_ato = DalSwitchDB.GetEDWAtomicContext())
            {
                var query = (from a in edw_ato.HIER_MEMBER_OPER.AsNoTracking()
                             join c in edw_ato.CENTER_LKP.AsNoTracking()
                                 on a.Clinic_Code equals c.Center_Code
                             where a.GMPI_Id == gmpi_id && byteFlag.Contains(a.Current_Enrollment_Flag.Value)
                             select c.Center_Wellmed_Owned_Flag).Distinct();
                if (query.Any(p => p == "Y"))
                {
                    return "Y";
                }

                return "N";
            }
        }

DalSwitchDB：

 /// 

    /// This class contains SwitchDB methods
    /// 
    public class DalSwitchDB
    {
        private static readonly log4net.ILog logger = log4net.LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
        private static IConnectionStringStorageContainer _connectionStringStorageContainer;

        /// 

        /// static Constructor of DalSwitchDB
        /// 
        static DalSwitchDB()
        {
            _connectionStringStorageContainer = ConnectionStringStorageFactory.CreateStorageContainer();
        }

public static EDW_AtomicEntities GetEDWAtomicContext()
        {
            logger.Info("ENTER " + System.Reflection.MethodBase.GetCurrentMethod().Name + DateTime.Now.ToString());

            string EDW_Atomic_cn = _connectionStringStorageContainer.GetConnectionString("DW02Atomic");
            EDW_AtomicEntities e_EDW_Atomic = new EDW_AtomicEntities(EDW_Atomic_cn);
            e_EDW_Atomic.Configuration.AutoDetectChangesEnabled = false;

            logger.Debug("Underlying EDW_Atomic SQL Connection String = " + EDW_Atomic_cn);
            logger.Info("EXIT " + System.Reflection.MethodBase.GetCurrentMethod().Name + DateTime.Now.ToString());

            return e_EDW_Atomic;
        }