做项目的时候一般是一个链接一个库，但是有时候碰上分库的需求场景，比如saas模式，a租户数据存在a数据库，b租户数据存在b数据库。那怎么来实现数据源的动态、灵活的切换呢？下面我们就来看看怎么实现。

实现思路
1、 web.config配置数据库连接串

  <add name="Bqool_ServiceEntities" connectionString="metadata=res://*/BqoolService.csdl|res://*/BqoolService.ssdl|res://*/BqoolService.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source={0};initial catalog={1};persist security info=True;user id=XXXXXX;password=XXXXX;MultipleActiveResultSets=True;App=EntityFramework&quot;" providerName="System.Data.EntityClient" />

2、注入ConnectionService

  //根据客户切换不同连接
RegisterScope(builder.RegisterType<ConnectionSchDbService>().As<IConnectionService>().WithParameter("sourceConnectionString", conn.Bqool_Service).Keyed<IConnectionService>(ConnectionType.Bqool_Service));

3、 获取动态生成的数据库链接

 public class ConnectionSchDbService : IConnectionService
    {
        private readonly string _sourceConnectionString;
        private readonly IAccountServiceRelService _accountServiceRel;
        private readonly ILoginStatusServices _loginStatusService;
        private bool? _isConnectUI;
        private readonly Logger _logger;

        public ConnectionSchDbService(
            string sourceConnectionString,
            IAccountServiceRelService accountServiceRel,
            ILoginStatusServices loginStatusService
               )
        {
            _sourceConnectionString = sourceConnectionString;
            _accountServiceRel = accountServiceRel;
            _loginStatusService = loginStatusService;

            _logger = LogManager.GetLogger(GetType().FullName);
        }

        public string GetDbConnectionString(string account = null)
        {
            var isConnectUI = false;
            if (_isConnectUI == true)
            {
                isConnectUI = true;
            }
            return SetConnection(account, isConnectUI);
        }

        public string GetServerName(string account = null)
        {
            var accountServiceRel = GetAccountServiceRelHistory(account);
            if (accountServiceRel != null)
            {
                return accountServiceRel.Server_Name;
            }
            return null;
        }

        public string GetServiceName(string account = null)
        {
            var accountServiceRel = GetAccountServiceRelHistory(account);
            if (accountServiceRel != null)
            {
                return accountServiceRel.Schedule_Name;
            }
            return null;
        }

        public void SetConnectionUI(bool isConnectUI)
        {
            _isConnectUI = isConnectUI;
        }

        private string SetConnection(string account, bool isConnectUI = false)
        {
            var environmentMode = CommonSetting.GetEnvironmentMode();
            var returnConnection = string.Empty;

            //Default Setting (Account Service Rel 还没建立的预设值)
            switch (environmentMode)
            {
                case EnvironmentMode.Dev: //开发
                    returnConnection = string.Format(_sourceConnectionString, "192.168.xxxx.xxxx", "Servicexxxx"); //开发环境使用 service1 ~ service3
                  xxxxx
                    break;

                xxxxxx

                default:
                    break;
            }
            // 堆代码 duidaima.com
            //连接到 Account_Service_Rel_History
            var accountServiceRel = GetAccountServiceRelHistory(account);
            if (accountServiceRel != null)
            {
                returnConnection = string.Format(_sourceConnectionString, accountServiceRel.Server_IP, accountServiceRel.Schedule_Name);
            }

            if (!System.Environment.UserInteractive && System.Web.HttpContext.Current == null) //Windows Service
            {
                _logger.Debug($"account:{account ?? _loginStatusService.GetAccount()}, isConnectUI:{isConnectUI}, conn:{StringTools.ReplaceRegex(returnConnection, @"user.*", "")}");
            }
            return returnConnection;
        }
  public interface IConnectionService
    {
        /// <summary>
        /// 取得资料库连接
        /// </summary>
        /// <param name="account">不填使用登入的 Account</param>
        /// <returns></returns>
        string GetDbConnectionString(string account = null);}

4 、定义访问的上下文DbContext

  public class DbContextFactory : IDbContextFactory
    {
        private readonly IConnectionService _connectionService;
        private readonly ILoginStatusServices _loginStatusServices;
        private readonly string _user;

        public DbContextFactory(IConnectionService connectionService, ILoginStatusServices loginStatusService)
        {
            _connectionService = connectionService;
            _loginStatusServices = loginStatusService;

            _user = loginStatusService.GetUserEmail();
        }

        private string _defaultConnectionString;
        private DbContext _dbContext;
        private SqlConnection _sqlConnection;
        private string _account;
        private string _serverName;
        private string _serviceName;

        public DbContext GetDbContext(string account = null)
        {
            var isResetConnection = IsResetConnection(account);
            if (_dbContext != null && isResetConnection)
            {
                DisposeDbContext();
            }
            if (_dbContext == null || isResetConnection)
            {                
                if (string.IsNullOrEmpty(_defaultConnectionString) || isResetConnection)
                {
                    _defaultConnectionString = _connectionService.GetDbConnectionString(account);
                }

                Type t = typeof(DbContext);
                _dbContext =
                    (DbContext)Activator.CreateInstance(t, _defaultConnectionString);
            }

            return _dbContext;
        }

5 、根据得到的上下文链接SqlConnection ，访问数据库

  public class GenericDapperRep : IGenericDapperRep
    {
        private IDbContextFactory _factory;
        protected string _serverName { get; set; }
        protected string _serviceName { get; set; }
        protected string _user { get; set; }
        protected SqlConnection _conn
        {
            get
            {
                return _factory.GetSqlConnection();
            }
        }

        public GenericDapperRep(IDbContextFactory factory)
        {
            _factory = factory;
            //factory.SetConnectionUI(false);
            SetDbContextFactory(factory);
        }

        //public GenericDapperRep(IDbContextFactory factory, bool isConnectUI)
        //{
        //    _factory = factory;
        //    factory.SetConnectionUI(isConnectUI);
        //    SetDbContextFactory(factory);
        //}

        private void SetDbContextFactory(IDbContextFactory factory)
        {
            if (factory == null)
            {
                throw new ArgumentNullException("factory");
            }
            //_conn = factory.GetSqlConnection();
            _serverName = factory.GetServerName();
            _serviceName = factory.GetServiceName();

            _user = factory.GetUser();
        }

        protected T DbQueryWrapper<T>(Func<T> doWork)
        {
            return _factory.DbQueryWrapper(doWork);
        }

        protected void Create(string sql, object data)
        {
            SetCreate(data);
            _conn.Execute(sql, data);
        }

总结一下
1、动态生成访问链接
2、定义访问的上下文DbContext

3、连上数据库操作