Records vs Classes vs Structs

C# / .NET Interview Notes

1. Big Picture

C# provides several ways to define custom types: class, struct, record, and record struct.

⭐ Quick Memory Trick:

Class → "WHO is this?" (Identity)

Struct → "WHAT value is this?" (Value)

Record → "Is this data equal to that data?" (Value-based equality)

2. Class

A class is a reference type.

class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

Person p1 = new() { Name = "John", Age = 30 };
Person p2 = p1;

p2.Name = "David";
Console.WriteLine(p1.Name);  // David (same object)

Typical use

Customer, Employee, Order
BankAccount, Vehicle
Service, Repository

3. Struct

A struct is a value type.

struct Point
{
    public int X;
    public int Y;
}

Point p1 = new() { X = 10, Y = 20 };
Point p2 = p1;

p2.X = 100;
Console.WriteLine(p1.X);  // 10 (copy, not same object)

4. Important: Are Structs Mutable?

Yes. A struct can be either mutable or immutable.

// Mutable struct
struct Point
{
    public int X;
    public int Y;
}

// Immutable struct
readonly struct Point
{
    public int X { get; }
    public int Y { get; }
}

⭐ Interview Point: Struct means value type. It does not mean immutable.

5. Record

A record is designed for data-oriented types with value-based equality.

record Person(string Name, int Age);

Person p1 = new("John", 30);
Person p2 = new("John", 30);

Console.WriteLine(p1 == p2);  // True (value-based equality)

6. Class Equality vs Record Equality

Class	Record
Reference equality (identity)	Value equality (data)
`p1 == p2` → False (different objects)	`p1 == p2` → True (same data)

7. Records and Immutability

Records encourage immutable patterns with with expressions:

Person p1 = new("John", 30);
Person p2 = p1 with { Name = "David" };

// p1 → John, 30 (unchanged)
// p2 → David, 30 (new copy)

⭐ Non-destructive mutation: "Create a copy, change selected properties, return new object."

8. Record Does NOT Automatically Mean Immutable

record Person
{
    public string Name { get; set; }  // Mutable!
}

var person = new Person();
person.Name = "John";  // Allowed

9. Record Class vs Record Struct

Type	Storage	Equality	with
`record`	Reference	Value-based	✅
`record struct`	Value	Value-based	✅
`readonly record struct`	Value (immutable)	Value-based	✅

10. Four Important Type Categories

C# Types ├── Reference Types │ ├── class │ └── record └── Value Types ├── struct └── record struct

11. Class vs Struct

Feature	Class	Struct
Type	Reference	Value
Assignment	Copies reference	Copies value
Identity	Usually important	Usually not
Inheritance	Supports class inheritance	Cannot inherit
Typical use	Entities	Small values

12. Class vs Record

Feature	Class	Record
Main concept	Identity	Data
Equality	Identity-oriented	Value-based
`with`	❌	✅
Good for	Entities	DTOs, messages, events

13. Choosing the Right Type

Does identity matter? → YES → class (Customer, Order, BankAccount) Is it primarily data where equality matters? → YES → record (DTO, API response, Event, Message) Is it a small value with no identity? → YES → struct (Point, Coordinates, Temperature)

⭐ Rule of thumb: Choose struct when data is value-oriented and short-lived, and when identity does not matter.

Is it a small value + record semantics? → record struct (Small value objects)

14. Memory: Don't Say "Struct = Stack"

⭐ Interview Answer: Don't say "Structs are stored on the stack and classes on the heap." The correct concept is value type vs reference type. Actual storage depends on context.

15. Boxing

int x = 10;
object obj = x;  // Boxing

int y = (int)obj;  // Unboxing

16. Real-World Example

// Order - has identity
class Order
{
    public long Id { get; set; }
    public decimal Quantity { get; set; }
}

// API response - data-oriented
record OrderResponse(long Id, decimal Quantity, string Status);

// Price - small value
readonly record struct Price(decimal Value);

17. Interview Questions

Q1. Is a struct mutable?

Answer: Yes. A struct describes a value type, not immutability.

Q2. Are records immutable?

Answer: Records are designed for immutable patterns, but aren't inherently immutable. A record can contain mutable properties.

Q3. Is a record a value type?

Answer: A normal record is a reference type. record struct is a value type.

Q4. Why use records for DTOs?

Answer: DTOs primarily represent data rather than object identity, so records' value-based equality and concise syntax make them a natural choice.

Q5. Are structs always stored on the stack?

Answer: No. Structs are value types, but their storage location depends on context.

18. Quick Revision

Concept	Remember
Class	Reference type, identity matters
Struct	Value type, value matters
Record	Reference type, value-based equality
Record Struct	Value type, value-based equality
`with`	Non-destructive mutation (records only)
Boxing	Value type → reference type

Easy Memory Tricks

Class → WHO? (Identity) Struct → WHAT? (Value) Record → DATA (Value-based equality) readonly struct → Value + Cannot change record struct → Value + Record semantics

19. One-Line Interview Answer

A class is a reference type typically used for identity-based entities, a struct is a value type typically used for small values, and a record is a data-oriented type that provides value-based equality and convenient immutable-style operations such as with.

📚 C# Master Notes