Multi Tenant SaaS Application

http://msdn.microsoft.com/en-us/library/aa479086.aspx
http://www.devx.com/webdev/Article/44977
http://dba.stackexchange.com/questions/16745/handling-growing-number-of-tenants-in-multi-tenant-database-architecture
http://www.ibm.com/developerworks/webservices/library/ws-middleware/

Factory Design Pattern

http://www.c-sharpcorner.com/UploadFile/db2972/factory-design-pattern/

Inversion of Control and Dependency Injection

http://www.c-sharpcorner.com/UploadFile/db2972/inversion-of-control-and-dependency-injection/