Differentiate between an Interface and an Abstract class

Interface:

• Interfaces are closely related to abstract classes that have all members abstract.
• All the methods of an interface must be virtual.
• A Class that implements an interface must provide concrete implementation of all the methods definition in an interface or else must be declared an abstract class.
• In C#, multiple inheritance is possible only through implementation of multiple interfaces.
•  An interface defines a contract and can only contains four entities viz methods, properties, events and indexes. An interface thus cannot contain constants, fields, operators, constructors, destructors, static constructors, or types.
• Also an interface cannot contain static members of any kind. The modifiers abstract, public, protected, internal, private, virtual, override is disallowed, as they make no sense in this context.
• Class members that implement the interface members must be publicly accessible.
• Interface increase security by hiding the implementation.

Abstract Class:

• At least one method of an abstract class must be an abstract method that means it may have concrete methods.
• Abstract class’s methods can’t have implementation only when declared abstract, otherwise they can have implementations and they have to be extended.
• Abstract class can implement more than one interfaces, but can inherit only one class.
• Abstract classes can only be derived once.
• Abstract class must override all abstract method and may override virtual methods.
• Abstract class can be used when implementing framework
• Abstract classes are an excellent way to create planned inheritance hierarchies and also to use as non-leaf classes in class hierarchies.