Making choice between Interface and Abstract Class

What is Interface

• An Interface is a group of constants and method declaration.
• .Net supports multiple inheritance through Interface.
• Interface states “what” to do, rather than “how” to do.
• An interface defines only the members that will be made available by an implementing object. The definition of the interface states nothing about the implementation of the members, only the parameters they take and the types of values they will return. Implementation of an interface is left entirely to the implementing class. It is possible, therefore, for different objects to provide dramatically different implementations of the same members.
• Example1, the Car object might implement the IDrivable interface (by convention, interfaces usually begin with I), which specifies the GoForward, GoBackward, and Halt methods. Other classes, such as Truck, Aircraft, Train or Boat might implement this interface and thus are able to interact with the Driver object. The Driver object is unaware of which interface implementation it is interacting with; it is only aware of the interface itself.
• Example2, an interface named IShape, which defines a single method CalculateArea. A Circle class implementing this interface will calculate its area differently than a Square class implementing the same interface. However, an object that needs to interact with an IShape can call the CalculateArea method in either a Circle or a Square and obtain a valid result.

• Practical Example

public interface IDrivable
{
void GoForward(int Speed);
}

public class Truck : IDrivable
{
public void GoForward(int Speed)
{
// Implementation omitted
}
}

public class Aircraft : IDrivable
{
public void GoForward(int Speed)
{
// Implementation omitted
}
}

public class Train : IDrivable
{
public void GoForward(int Speed)
{
// Implementation omitted
}
}


Extra

• Each variable declared in interface must be assigned a constant value.
• Every interface variable is implicitly public, static and final.
• Every interface method is implicitly public and abstract.
• Interfaces are allowed to extends other interfaces, but sub interface cannot define the methods declared in the super interface, as sub interface is still interface and not class.
• If a class that implements an interface does not implements all the methods of the interface, then the class becomes an abstract class and cannot be instantiated.
• Both classes and structures can implement interfaces, including multiple interfaces.

Making choice between Interface and Abstract Class

In which Scenario you will go for Interface or Abstract Class?
Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces
do not provide implementation. They are implemented by classes, and defined as separate entities from
classes. Even though class inheritance allows your classes to inherit implementation from a base class, it
also forces you to make most of your design decisions when the class is first published.

Abstract classes are useful when creating components because they allow you specify an invariant level
of functionality in some methods, but leave the implementation of other methods until a specific
implementation of that class is needed. They also version well, because if additional functionality is
needed in derived classes, it can be added to the base class without breaking code.