6 question(s)
What are the four pillars of OOP in C#?
Beginner
Encapsulation (hide state behind members), Inheritance (derive types), Polymorphism (one interface, many implementations) and Abstraction (expose essentials, hide detail).
public abstract class Shape { public abstract double Area(); }
public class Square(double s) : Shape { public override double Area() => s*s; }
Real-world example
A renderer iterates List<Shape> and calls Area() without knowing concrete types.
How do properties provide encapsulation?
Beginner
Properties expose get/set accessors over a private field, letting you validate on set and compute on get while keeping the field hidden.
public int Age { get; private set; }
public decimal Discount { get => _d; set => _d = Math.Clamp(value,0,100); }
Real-world example
Clamping a discount to 0-100 so no caller can set an invalid value.
What is the difference between overriding and hiding (override vs new)?
Intermediate
override replaces a virtual/abstract base member and uses run-time (polymorphic) dispatch; new hides the base member and dispatch depends on the compile-time type, which is usually a bug.
class B { public virtual void Go()=>Console.Write("B"); }
class D : B { public override void Go()=>Console.Write("D"); }
Real-world example
Overriding lets a PdfReport customise Render() while base code calls it polymorphically.
What does the sealed keyword do and why use it?
Intermediate
sealed stops a class being inherited or a member being further overridden. It can protect invariants and lets the JIT devirtualise calls for speed.
public sealed class Logger { }
Real-world example
Sealing a security-sensitive type so no subclass can weaken its behaviour.
What is composition over inheritance and when do you prefer it?
Advanced
Composition builds behaviour by holding other objects rather than inheriting; it avoids deep, rigid hierarchies and the fragile base-class problem. Prefer it unless there is a true is-a relationship.
class Car { private readonly Engine _engine; public Car(Engine e)=>_engine=e; }
Real-world example
Injecting an IPaymentGateway into an OrderService instead of subclassing a base service.
How do virtual method tables enable polymorphism, and what is the cost?
Advanced
Each type with virtual members has a method table; a virtual call looks up the actual implementation at run time via the object's type. It costs an indirection versus a direct/sealed call.
Shape s = new Square(3);
double a = s.Area(); // resolved to Square.Area at run time
Real-world example
Hot loops sometimes seal types or use generics to let the JIT remove virtual dispatch.