using multiple DBs with single application

Any design pattern? 1st answer is Factory Pattern and Repository pattern for Encapsulating CRUD operations....Also use DI for a loosely coupled communication between DAL and other layers. But you don't need to do all these as there are built-in ORM tools available to achieve this. e.g. Entity Framework, NHibernate etc...Here one great article of using EF, which is meant to interact with Oracle, MySQL, PostgreSQL or SQLite rather than Microsoft SQL Server https://www.infoq.com/articles/multiple-databases. It used XML mapping. You please google for more info here...

 

But on learning side, always check these patterns and standards. EF source code is available to check here, https://github.com/aspnet/EntityFramework6