Interfaces in C# are another way of achieving polymorphism apart from inheritance (via abstract classes 
). They create a contract which requires the classes implementing the interface to provide implementation of the methods defined in the interface.
An interface cannot have implementation and data (fields) within it. When an interface requires the derived class to have certain data, it uses a property rather than a field and the property does not contain any implementation as part of declaration.
Defining private or protected members in an interface would defeat the purpose of the interface contract, thus C# does not allow access modifiers on interface members and instead it automatically defines them as public.
A class can implement multiple interfaces.
When a class declares that it implements an interface, all members of the interface must be implemented. Even at least with the NotImplementedException, but from a compiler’s perspective it should provide an implementation.
An interface can never be instantiated and thus cannot even have constructors or finalizers. Interface instances are available only through types that implement them.
An interface cannot have static members as polymorphism won’t be of any value if there is no instance of the implementing type.
Methods within an interface cannot have abstract modifier as they are automatically abstract.
