What?
The Common Reuse Principle (CRP) states: ”The classes in a component are reused together. If you reuse one of the classes in a component, you reuse them all.”
In the last post I wrote about the Reuse/Release Equivalence Principle (REP). It says that reusable components must be releasable components, with everything that comes with making it so. I finished with writing that classes should have a common theme or use case.
CRP emphasizes this, by saying that the classes that make up a component should be inseparable. It should be hard or impossible to reuse only some of the classes of a component. The using component should be (directly or indirectly) be using all of them.
Why?
Components in .NET are made from assemblies carried in a DLL. If one component uses a class in another component, even if it only uses a single method in a single class, it has a strong dependency to that component.
This means that every time someone makes a change to used component, even in code that the using component do not use and don’t care about, a new version of the used DLL is created and the using component must be re-validated against that new version.
This is a waste of resources and effort and should be avoided
How?
CRP says something about which classes to put in a component, but mostly it says which classes to not put in the component. The classes that make up a component should be tightly coupled and depend heavily on each other. It should not be possible to separate them easily. If a class does not fit into this description it does not belong in the component and should be put elsewhere.