Agile Principles, Patterns, and Practices in C#

© Jennifer M. Kohnke

Nevermore Let the great interests of the State depend Upon the thousand chances that may sway A piece of human frailty

Sir Thomas Noon Talfourd (17951854)

Over the years, many have questioned why I use the word inversion in the name of this principle. The reason is that more traditional software development methods, such as structured analysis and design, tend to create software structures in which high-level modules depend on low-level modules and in which policy depends on detail. Indeed, one of the goals of these methods is to define the subprogram hierarchy that describes how the high-level modules make calls to the low-level modules. The initial design of the Copy program in Figure 7-1 is a good example of such a hierarchy. The dependency structure of a well-designed object-oriented program is "inverted" with respect to the dependency structure that normally results from traditional procedural methods.

Consider the implications of high-level modules that depend on low-level modules. It is the high-level modules that contain the important policy decisions and business models of an application. These modules contain the identity of the application. Yet when these modules depend on the lower-level modules, changes to the lower-level modules can have direct effects on the higher-level modules and can force them to change in turn.

This predicament is absurd! It is the high-level, policy-setting modules that ought to be influencing the low-level detailed modules. The modules that contain the high-level business rules should take precedence over, and be independent of, the modules that contain the implementation details. High-level modules simply should not depend on low-level modules in any way.

Moreover, it is high-level, policy-setting modules that we want to be able to reuse. We are already quite good at reusing low-level modules in the form of subroutine libraries. When high-level modules depend on low-level modules, it becomes very difficult to reuse those high-level modules in different contexts. However, when the high-level modules are independent of the low-level modules, the high-level modules can be reused quite simply. This principle is at the very heart of framework design.