Object-oriented programming

Programmers refer to the problem you are trying to solve and all the information you know that relates to your problem as the problem domain. For example, if you are writing a program to manage the inventory and sales of a company, the problem domain would include everything you know about how the company acquires and manages inventory, makes sales, handles the income from sales, tracks sales figures, and so forth. The sales manager and the stock room manager would be problem domain experts who can help you understand the problem domain.

• State: a programmer refers to the current conditions and values of an object as that object's state. For example, you might have an object representing a customer. The customer's state includes the customer's address, phone number, email, as well as the customer's credit rating, recent purchase history, and so forth.
• Capabilities: The customer has many capabilities, but a developer cares only about modeling those that are relevant to the problem domain. Thus a customer object might be able to buy an item, return an item, increase his credit rating, and so forth.
• Responsibilities: Along with capabilities come responsibilities. The customer object is responsible for managing its own address. In a well-designed program, no other object needs to know the details of the customer's address. The address might be stored as data within the customer object, or it might be stored in a database, but it is up to the customer object to know how to retrieve and update his own address.

• Good metaphors can be very powerful. The art of object-oriented programming is really the art of conceiving of good metaphors.
• A good object-oriented design is an accurate model of the problem you are trying to solve. Your design choices influence not only how you solve the problem, but in fact they influence how you think about the problem. A good design, like a good model, allows you to examine the relevant details of the problem without confusion.
• The theory behind object-oriented programming is that for computer programs to accurately model the world, the programs should reflect this human tendency to think about individual things and types of things. In C# you do that by creating a class to define a type and creating an object to model a thing.
• In a good object-oriented program, you will design objects that represent things in your problem domain. You will then divide the work of the program among your objects, assigning responsibility to objects based on their ability.