class design

What are the facts need to be checked when designing a class .NET . Here are a few. 1. Can you inherit from an existing class? 2. Does this class need to be concrete, an interface , an abstract class or static class? 3. IS this class part of a pattern or a standalone? 4. Does this need to be part of a library ? What is the namespace? 5. What properties and methods it needs to expose and why? When to use Interface ? a. When creating a standalone project which can be changed at will, use an interface in preference to an abstract class; because, it offers more design flexibility. b. Use interfaces to introduce polymorphic behavior without subclassing and to model multiple inheritance—allowing a specific type to support numerous behaviors. c. Use an interface to design a polymorphic hierarchy for value types. d. Use an interface when an immutable contract is really intended. e. A well-designed interface defines a very specific range of functionality. Split up interfaces that contain unrelated functionality f. Need to think about future business changes ( extensibility) if this is a business object. When to use an abstract class ? a. When creating a class library which will be widely distributed or reused—especially to clients, use an abstract class in preference to an interface; because, it simplifies versioning. This is the practice used by the Microsoft team which developed the base class library (com was designed around interfaces.) b. Use an abstract class to define a common base class for a family of types. c. Use an abstract class to provide default behavior. d. Subclass only a base class in a hierarchy to which the class logically belongs. 6. When deciding between property and method: If the purpose is to set or retrieve a piece of data define a property. If it is to perform a general action then define a method.