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•  Abstract methods do not have an implementation.
•  Abstract methods are implicitly virtual.
•  If a class has abstract methods (declared or inherited) it must be abstract itself.
•  One cannot create objects of an abstract class.
   

• Interface = purely abstract class; only signatures, no implementation.
• May contain methods, properties, indexers and events (no fields, constants, constructors, destructors, operators, nested types).
• Interface members are implicitly public abstract (virtual).
• Interface members must not be static.
• Classes and structs may implement multiple interfaces.
• Implemented by classes and structs
• A class can inherit from a single base class, but can implement multiple interfaces.
• A struct cannot inherit from any type, but can implement multiple interfaces.
• Every interface member (method, property, indexer) must be implemented or inherited from a base class.
• Implemented interface methods must not be declared as override.
• Implemented interface methods can be declared as virtual or abstract (i.e. an interface can be implemented by an abstract class).
• If subclasses of MyClass should be able to override Add() it must be declared as virtual (although Add() is already implicitly virtual in IList).
• Interfaces can extend other interfaces.
   

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