Constructors: default, parameterised, static, private, chaining (OOP Concepts in C#)

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public class Logger
{
    public static readonly DateTime LoadedAt;
    static Logger() { LoadedAt = DateTime.UtcNow; }      // static ctor: once per type, before first use

    public string Name { get; }
    public Logger() : this("default") { }                // chaining to another ctor
    public Logger(string name) { Name = name; }
}

public class Settings
{
    private Settings() { }                               // private ctor: no outside 'new'
    public static Settings Load() => new Settings();     // factory method / Singleton
}

public class Admin : User
{
    public Admin(string email) : base(email) { }         // call base constructor
}

• If you write no constructor, C# provides a parameterless one. Writing any constructor removes it.
• Order: static fields → static ctor (once) → instance field initialisers → base ctor → derived ctor body.
• Copy constructor: a constructor taking another instance of the same type (records generate one).

Interview questions on OOP Concepts in C

What are the four pillars of OOP? Explain each with an example.

1. Encapsulation: private state with a controlled public API. BankAccount keeps _balance private and exposes Deposit() that rejects negative amounts.
2. Abstraction: expose what, hide how. IPaymentGateway.ChargeAsync() hides HTTP calls, signatures and retries.
3. Inheritance: a derived class reuses and extends a base. Contractor : Employee reuses Name and Describe().
4. Polymorphism: the same call behaves differently by type. employee.MonthlyPay() runs the Permanent or Contractor override at runtime.

What is the difference between a class and an object?

A class is a type definition (blueprint) describing fields, properties and methods; it exists in code and metadata. An object is an instance of that class created at runtime with new, occupying memory with its own state. One class, many objects.

What is the difference between method overloading and method overriding?

Overloading	Overriding
Same name, different parameters	Same signature in a derived class
Same class (or derived)	Requires virtual/abstract + override
Resolved at compile time	Resolved at runtime by the object's type
Return type alone can't differ	Return type the same (covariant returns allowed since C# 9)

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Next in the course: Access modifiers →