Inversion of Control (IoC) is a very important practice to make code testable. It usually relies on defining interfaces for each external dependency. Various frameworks and techniques, i.e. Dependency Injection (DI), Service Locator, exist to bind the interface implementations to the components. In this blog post, we will *not* focus on this aspects of IoC but rather on it’s main building block: interfaces. This article describes how Contracts for interfaces improves IoC… regardless of which DI framework/technique you are using. The Contracts for interfaces are a feature of the Code Contracts for .Net tool.