When to use Interface and Abstract Class

• Interface members cannot have a definition
• All interface members should be implemented by the derived class
• An abstract class can have an instance field in it. The derived classes can assess this field through the  base syntax. This is the key difference between abstract and interfaces.

Abstract class can contain non abstract method. When we want that all subclass must implement all methods of base class ;In such situation use interface.In other hand when we want only some method of base class in our subclass then use base class as abstract class because abstract class can contain non-abstract method. Interfaces are used when we want classes should follow the rules strictly. Rules like naming coventions e.g. if in customer class one developer add method AddCustomer other developer while implementing Invoice class could write method name InvoiceAdd and also could change the Signiture rules. for addition of customer we have at client code as Cutomer.AddCustomer(A,B,C); and Invoice.InvoiceAdd(A,B) To maintain uniformality if we declare interface and define method Add(A,B) in that interface and implement that interface in both the classes then the code will be uniform all over the classes where ever we have implemented interface and also all concrete classes must contain methods which are defined in the interfaces. Interface: –> If your child classes should all implement a certain group of methods/functionalities but each of the child classes is free to provide its own implementation then use interfaces. For e.g. if you are implementing a class hierarchy for vehicles implement an interface called Vehicle which has properties like Colour MaxSpeed etc. and methods like Drive(). All child classes like Car Scooter AirPlane SolarCar etc. should derive from this base interface but provide a seperate implementation of the methods and properties exposed by Vehicle. –> If you want your child classes to implement multiple unrelated functionalities in short multiple inheritance use interfaces. For e.g. if you are implementing a class called SpaceShip that has to have functionalities from a Vehicle as well as that from a UFO then make both Vehicle and UFO as interfaces and then create a class SpaceShip that implements both Vehicle and UFO . Abstract Classes –> When you have a requirement where your base class should provide default implementation of certain methods whereas other methods should be open to being overridden by child classes use abstract classes. For e.g. again take the example of the Vehicle class above. If we want all classes deriving from Vehicle to implement the Drive() method in a fixed way whereas the other methods can be overridden by child classes. In such a scenario we implement the Vehicle class as an abstract class with an implementation of Drive while leave the other methods / properties as abstract so they could be overridden by child classes. –> The purpose of an abstract class is to provide a common definition of a base class that multiple derived classes can share. For example a class library may define an abstract class that is used as a parameter to many of its functions and require programmers using that library to provide their own implementation of the class by creating a derived class. Use an abstract class * 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.) * Use an abstract class to define a common base class for a family of types. * Use an abstract class to provide default behavior. * Subclass only a base class in a hierarchy to which the class logically belongs. Use an interface * 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. * Use interfaces to introduce polymorphic behavior without subclassing and to model multiple inheritance—allowing a specific type to support numerous behaviors. * Use an interface to design a polymorphic hierarchy for value types. * Use an interface when an immutable contract is really intended. * A well-designed interface defines a very specific range of functionality. Split up interfaces that contain unrelated functionality.