S - Single Responsibility Principle
O - Open/Closed Principal
L - Liskov Substitution Principal
I - Interface Segregation Principal
D - Dependency Inversion Principal
The Single Responsibility Principal states that a
class should only be responsible for doing one type of thing, not multiple
things. For example an OrderCalculator class should not contain methods that do
calcuations and sending emails. Instead any other responsibilities should be
separated out and placed into another class.
The Open/Closed Principal states that a design
should be open for extension and closed to modification. It means that you
should not have to edit an existing class in order to add new functionality.
Instead you should be able to add a new class to add new functionality. That
way by adding a new class you are not touching any existing code and this means
that the chance of breaking something is significantly reduced.
The Liskov Substitution Principal states that
subtypes must be substitutable for their base types.
The Interface Segregation Principal tells us that we shouldn't create fat interfaces.
Instead we should create lots of smaller interfaces instead. For example if you
have a client that only need to do VerifyLogin(username,
password) then why create an interface
with 100 methods on it?
The Dependency Inversion Principal allows us to
create loosely coupled components.