With new techniques and patterns interface plays a key role in application architecture. Interface makes application extendable like defining file upload interface and implementing based on file system, Azure Blob storage, Amazon S3. At starting we might be implementing based on Azure Blob but later we might move to Windows based file system and so on. Ideally we create interface based on need and start implementing actual default implementation class. Many a times at starting of implementation there is one to one mapping between Interface and Class. Like from above example File upload interface and the initial or default class implementation that we design and with time it will get extended. In this article, we will try to create interface based on default class implementation. This is not at all recommended in Test Driven Design (TDD) where we test the application before actual code implementation but I feel sometimes and in some situations it is okay do that and test straig