The Open-Closed Principle states that modules should be open for extension but closed for modification. Simply stated, this means that we should not have to change our code every time a requirement or a rule changes. Our code should be extensible enough that we can write it once and not have to change it later just because we have new functionality or a change to a requirement. If you consider the number of times that code is changed after an application makes its first ‘release’ and you begin to quantify the actual time (and expense) associated with repeated changes to code over the life of the application, the actual development cost is probably not going to surpass the ongoing maintenance costs.