What is the difference between an interface and abstract class?

Question

What exactly is the difference between an interface and abstract class?

Answer 1

Key Points:

• Abstract class can have property, Data fields ,Methods (complete / incomplete) both.
• If method or Properties define in abstract keyword that must override in derived class.(its work as a tightly coupled functionality)
• If define abstract keyword for method or properties in abstract class you can not define body of method and get/set value for properties and that must override in derived class.
• Abstract class does not support multiple inheritance.
• Abstract class contains Constructors.
• An abstract class can contain access modifiers for the subs, functions, properties.
• Only Complete Member of abstract class can be Static.
• An interface can inherit from another interface only and cannot inherit from an abstract class, where as an abstract class can inherit from another abstract class or another interface.

Advantage:

• It is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.
• If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
• If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.
• Its allow fast execution than interface.(interface Requires more time to find the actual method in the corresponding classes.)
• It can use for tight and loosely coupling.

find details here... http://pradeepatkari.wordpress.com/2014/11/20/interface-and-abstract-class-in-c-oops/