Interfaces provide a form of multiple inheritance. A class 
can extend only one other class. 
Interfaces are limited to public methods and constants with 
no implementation. Abstract classes can have a partial 
implementation, protected parts, static methods, etc. 
A Class may implement several interfaces. But in case of 
abstract class, a class may extend only one abstract class. 
Interfaces are slow as it requires extra indirection to to 
find corresponding method in in the actual class. Abstract 
classes are fast.

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(1)       An abstract class may contain complete or 
incomplete methods. Interfaces can contain only the 
signature of a method but no body. Thus an abstract class 
can implement methods but an interface can not implement 
methods.
(2)         An abstract class can contain fields, 
constructors, or destructors and implement properties. An 
interface can not contain fields, constructors, or 
destructors and it has only the property's signature but no 
implementation.
(3)         An abstract class cannot support multiple 
inheritance, but an interface can support multiple 
inheritance. Thus a class may inherit several interfaces 
but only one abstract class.
(4)         A class implementing an interface has to 
implement all the methods of the interface, but the same is 
not required in the case of an abstract Class.
(5)       Various access modifiers such as abstract, 
protected, internal, public, virtual, etc. are useful in 
abstract Classes but not in interfaces. 
(6)         Abstract classes are faster than interfaces.

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ABSTRACT CLASS:
1) we can't create object of abstract class.
2) we can declare a variable.
3) we can make normal method and abstract method.
4) we can't make method body with abstract method.
5) in derived class we can use abstract method by override  
    keyword.
6) we can't use multiple inheritance in abstract class.
7) Absrtact class can be extended by only one class;
   Interface can be implemented by more than one class;  
   ie. class can inherit only one abstract class


8)Abstract classes can inherit interfaces while interfaces 
      cannot inherit abstract classes
   but class can implement more than one interface.
9)  The members of the interface are public with no
    implementation. Abstract classes can have protected   
    parts,static methods, etc.

10)An abstract class can contain fields, 
constructors, or destructors and implement properties. An 
interface can not contain fields, constructors, or 
destructors and it has abstractmethod,property's signature 
but no implementation etc.