Roadmap › 1. .NET › Step 10

OOP

OOP Concepts in C#

Classes and objects, the four pillars (encapsulation, abstraction, inheritance, polymorphism), interfaces vs abstract classes, composition and object relationships.

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Four pillars in one line each

Encapsulation — private data, public methods that guard rules.
Abstraction — interfaces/abstract classes expose what, hide how.
Inheritance — Derived : Base reuses and extends ("is-a").
Polymorphism — overloading (compile time) and overriding (runtime).

Interface vs abstract class

	Interface	Abstract class
Inherit many?	Yes	No (one)
State/fields	No	Yes
Constructor	No	Yes
Default implementation	Yes (C# 8+)	Yes
Best for	Contracts, DI, mocking	Shared base code

Keywords

Keyword	Meaning
`virtual`	May be overridden
`abstract`	Must be overridden; no body
`override`	Replaces base virtual/abstract
`sealed`	No further inheritance/override
`new`	Hides a base member (no polymorphism)
`base`	Access base members/constructor
`this`	Current instance / chain constructors
`static`	Belongs to the type

Relationships

Association: uses (independent lifetimes)
Aggregation: has-a (part survives)
Composition: owns-a (part dies with whole)
Inheritance: is-a
Prefer composition; inherit only for true substitutable "is-a".

Constructor order

Static field initialisers → static ctor (once) → instance field initialisers (derived, then base) → base ctor body → derived ctor body.

Equality checklist

Override Equals(object), implement IEquatable<T>, override GetHashCode with HashCode.Combine.
Or use record for value equality for free.