What is the difference between struct and class in C# with performance comparison?

Introduction

In C# and .NET development, one of the most commonly asked and practically important questions is the difference between struct and class. While both are used to define custom data types, their behavior in memory, performance, and real-world usage is significantly different.

If misunderstood, choosing the wrong type can lead to performance issues, memory overhead, or unexpected bugs in large-scale applications.

In this article, we will deeply explore struct vs class in C#, including memory allocation, performance comparison, real-world scenarios, and when to use each.

What is a Class in C#?

A class in C# is a reference type. This means that when you create an object of a class, memory is allocated on the heap, and variables store references (addresses) to that memory.

Example

class Person
{
    public string Name;
}

Person p1 = new Person();
p1.Name = "Rahul";

Person p2 = p1;
p2.Name = "Amit";

Output Behavior

Both p1 and p2 will have Name = "Amit" because they point to the same memory location.

Real-World Analogy

Think of a class as a Google Drive file link. Multiple people can access and modify the same file using the same reference.

What is a Struct in C#?

A struct in C# is a value type. It stores data directly, usually on the stack, and copies values when assigned.

Example

struct Person
{
    public string Name;
}

Person p1 = new Person();
p1.Name = "Rahul";

Person p2 = p1;
p2.Name = "Amit";

Output Behavior

p1.Name remains "Rahul" because struct creates a copy of data.

Real-World Analogy

Think of struct like a printed document. If you copy it, each person gets their own separate version.

Memory Allocation: Stack vs Heap

Understanding memory behavior is key to performance optimization in .NET applications.

Class → Stored in Heap
Struct → Stored in Stack (in most cases)

Why This Matters

Heap allocation is slower and requires garbage collection
Stack allocation is faster and automatically managed

Performance Comparison: Struct vs Class

Feature	Struct	Class
Type	Value Type	Reference Type
Memory	Stack (usually)	Heap
Speed	Faster (small data)	Slower due to GC
Copy Behavior	Copies value	Copies reference
Null Support	Not allowed (except nullable)	Allowed
Inheritance	Not supported	Supported

Real-World Performance Scenario

Scenario 1: High-Frequency Data (Struct Preferred)

Imagine a game engine calculating positions of thousands of objects per frame.

Using struct:

Faster allocation
Less GC pressure
Better performance

Scenario 2: Complex Business Object (Class Preferred)

For example, a User entity in an e-commerce system:

Requires inheritance
Needs shared references
Large data structure

Using class is better.

When to Use Struct in C#

Use struct when:

Object size is small (typically < 16 bytes)
Data is immutable
No need for inheritance
High-performance scenarios (games, real-time systems)

When to Use Class in C#

Use class when:

Object is large or complex
Requires inheritance or polymorphism
Needs to be shared across multiple components

Common Mistakes Developers Make

Mistake 1: Using Struct for Large Objects

This leads to performance degradation because copying large structs is expensive.

Mistake 2: Assuming Struct Always Uses Stack

Structs can be stored in heap if used inside a class.

Before vs After Understanding

Before:

Random use of class and struct
Performance issues in large applications

After:

Optimized memory usage
Better performance tuning
Cleaner architecture decisions

Advantages of Struct

Faster memory allocation
Reduced garbage collection
Better for small, immutable data

Disadvantages of Struct

No inheritance
Copy overhead for large data
Limited flexibility

Advantages of Class

Supports inheritance and polymorphism
Flexible and scalable
Suitable for complex systems

Disadvantages of Class

Heap allocation overhead
Garbage collection impact

Summary

Understanding the difference between struct and class in C# is essential for writing efficient and scalable .NET applications. Structs are ideal for small, high-performance scenarios where value copying is beneficial, while classes are better suited for complex, reference-based systems requiring flexibility and shared state. Choosing the right type based on memory behavior, performance needs, and application design can significantly improve your application's efficiency and maintainability.