Encapsulation bundles data with its methods and hides the internals, and inheritance builds a new class from an existing one to reuse its code. Polymorphism lets code use different types through one common type, and abstraction shows only the essential features.
C# has a language feature for each pillar:
For encapsulation, access modifiers like private, plus properties.
For inheritance, a single base class, as in class Car : Vehicle.
For polymorphism, virtual and override methods and interfaces.
For abstraction, abstract classes and interfaces.
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Likely follow-ups
How is encapsulation different from abstraction? They both sound like hiding details.Encapsulation guards a class's state behind its members. Abstraction picks which operations callers see. Use one class to show both.
Why does C# allow only one base class but let a class implement many interfaces?Mention the diamond problem, then explain that interfaces let a class take on several types without inheriting state from several bases.
What you can say
The four pillars are encapsulation, inheritance, polymorphism and abstraction, and C# has a language feature for each one.
Encapsulation bundles data with the methods that use it and hides the internals, so in C# I keep fields private and expose properties.
Inheritance builds a new class from an existing one to reuse its code, like class Car : Vehicle, and a C# class gets a single base class.
Polymorphism lets code work with different types through one common type, using virtual and override methods or interfaces.
Abstraction shows only the essential features, through abstract classes and interfaces, so callers depend on what a type does rather than how.
The pillar I'm most careful with is inheritance, because deep hierarchies couple classes tightly, so I often use interfaces and composition instead.
Weak answers to avoid
Recites four definitions with no codeInterviewers want to see you apply the ideas. Tie each pillar to a C# feature, like private fields with properties or virtual and override, and give one short example.
Treats encapsulation and abstraction as the sameBoth hide something, but encapsulation guards a class's state and abstraction picks which operations callers see. Blurring them sounds like memorized terms.
Says C# supports multiple inheritanceA C# class has exactly one base class. It can implement many interfaces, and that is how it takes on several types.
Presents inheritance as the default way to reuse codeDeep hierarchies tie subclasses to base class details. Say you use inheritance for real is-a links and interfaces or composition otherwise.
Interview lens
Likely follow-ups, what you can say, and the weak answers to avoid.
