🧠 Classes vs Structs in C#: When to Use Each?

C# gives developers the flexibility to choose between classes and structs, but many beginners (and even seasoned pros) often ask: "Which one should I use and when?" This question becomes even more important when performance, memory, and code structure matter.

📘 Introduction to Classes and Structs

What is a Class in C#?

A class in C# is a reference type. That means when you create an object of a class, you're creating a reference to that object in memory. Any time you assign that object to another variable or pass it to a method, you're just copying the reference — not the actual data.

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

Person p1 = new Person() { Name = "John", Age = 30 };
Person p2 = p1;
p2.Age = 40;
Console.WriteLine(p1.Age); // Output: 40
    
  

What is a Struct in C#?

A struct in C# is a value type. When you assign a struct to a new variable, it creates a copy of the data, not a reference. This leads to completely independent objects.

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

Point pt1 = new Point() { X = 1, Y = 2 };
Point pt2 = pt1;
pt2.X = 10;
Console.WriteLine(pt1.X); // Output: 1
    
  

⚖️ Core Differences Between Classes and Structs

Value Type vs Reference Type

Feature	Class	Struct
Type	Reference	Value
Stored in	Heap	Stack
Default behavior on assignment	Shallow copy (reference)	Deep copy (value)
Inheritance	Yes	No
Constructor	Custom allowed	Must init all fields
Null assignment	Allowed	Not allowed (unless nullable)

Memory Allocation and Performance

Memory and performance can be significantly affected by your choice. Structs live on the stack, making them ideal for small, short-lived objects. But be cautious — structs get boxed when cast to object, reducing performance.

📌 Use Cases: When to Use Classes

Ideal Scenarios for Classes

• You need to implement inheritance or polymorphism
• Your object has complex logic or relationships
• It needs to be shared and mutable
• You care about identity (e.g., multiple references to the same object)

Real-World Example of Using Classes

public class Car {
    public string Make { get; set; }
    public string Model { get; set; }
    public decimal Price { get; set; }

    public void DisplayInfo() {
        Console.WriteLine($"{Make} {Model} - ${Price}");
    }
}
    
  

Car car1 = new Car() { Make = "Toyota", Model = "Corolla", Price = 20000 };
Car car2 = car1;
car2.Price = 22000;
car1.DisplayInfo(); // Output: Toyota Corolla - $22000
    
  

🧷 Use Cases: When to Use Structs

Best Situations for Structs

• Object is small and used frequently
• Immutable behavior is preferred
• No need for polymorphism
• It’s used in performance-critical paths (e.g., graphics, math)

Real-World Example of Using Structs

public struct Pixel {
    public int X;
    public int Y;

    public Pixel(int x, int y) {
        X = x;
        Y = y;
    }
}
    
  

Pixel a = new Pixel(10, 20);
Pixel b = a;
b.X = 50;
Console.WriteLine(a.X); // Output: 10
    
  

🔁 Copy Behavior: Deep Copy vs Shallow Copy

How Copying Works with Structs

Structs in C# are value types, which means when you assign one struct variable to another, it creates a deep copy of the object. The term “deep copy” in this context doesn’t mean it copies nested objects recursively (unless you implement that), but rather, the actual values are duplicated. This is fundamentally different from how classes behave.

Let’s illustrate this:

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

Now use the struct:

Coordinates coord1 = new Coordinates { X = 5, Y = 10 };
Coordinates coord2 = coord1;
coord2.X = 20;

Console.WriteLine(coord1.X); // Output: 5
Console.WriteLine(coord2.X); // Output: 20

Here, coord2 is a completely separate copy from coord1. Changes made to one do not affect the other. This behavior is particularly useful when you want to avoid unintended side effects, especially in multi-threaded or high-performance environments. Since each copy is separate, there’s no risk of modifying shared state unless explicitly passed as a reference.

How Copying Works with Classes

Unlike structs, classes in C# are reference types. This means when you assign a class variable to another, you're only copying the reference — not the object’s actual data. Both variables point to the same memory location.

public class Settings {
    public int Volume;
    public int Brightness;
}

Let’s test copying behavior:

Settings set1 = new Settings { Volume = 50, Brightness = 70 };
Settings set2 = set1;
set2.Brightness = 90;

Console.WriteLine(set1.Brightness); // Output: 90

As you can see, both set1 and set2 refer to the same object. Changing one affects the other. This is useful when you want multiple parts of your application to refer to and modify the same data — but it can lead to bugs if not managed properly.

If you want to make an actual deep copy of a class, you need to implement it manually or use serialization or cloning techniques.

⚙️ Performance Considerations in Real Projects

When Performance Matters — Use Structs

In high-performance scenarios, such as game development, simulations, or mathematical computations, using structs can drastically improve execution speed. Why? Because structs are allocated on the stack, not the heap. Stack allocation is faster, and structs are automatically deallocated once they go out of scope, avoiding garbage collection (GC) delays.

Consider an application processing millions of 3D points. Using a class for this would create millions of heap allocations, putting strain on the GC. Structs eliminate that overhead and are more cache-friendly — a bonus in CPU-intensive loops.

public struct Vector3 {
    public float X, Y, Z;

    public Vector3(float x, float y, float z) {
        X = x; Y = y; Z = z;
    }
}

Use structs for:

• Large arrays of simple data
• Pixel manipulation in graphics software
• Physics simulations (e.g., velocity, position vectors)

When to Prefer Classes for Performance

It might sound contradictory, but sometimes classes are better for performance, especially when:

• You frequently pass objects around by reference
• You need polymorphism (virtual methods)
• Structs would be boxed/unboxed repeatedly

For example, when you're using LINQ or working with object-oriented patterns like dependency injection, classes are easier to manage and more memory efficient than boxed structs.

Structs larger than 16 bytes may start to slow things down due to copying overhead. In such cases, classes outperform them — especially in large applications with frequent object passing.

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💡 Design Guidelines: Microsoft’s Official Advice

What Microsoft Recommends

According to Microsoft’s design guidelines, here’s when to use a struct:

• It logically represents a single value (like a number or coordinate)
• It is small (typically less than 16 bytes)
• It is immutable after creation
• It does not require inheritance or reference semantics

If the type doesn’t meet all these criteria, it should probably be a class. Microsoft strongly advises against using mutable structs in public APIs, as it can lead to confusing and error-prone behavior.

Examples of Common Structs in .NET

The .NET framework uses structs in many key areas:

• System.DateTime – Represents date and time
• System.TimeSpan – Represents a time interval
• System.Guid – Globally unique identifier
• System.Drawing.Point – Represents X, Y coordinates

These are all excellent use cases for structs: small, immutable, and logically value types.

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📊 Benchmark: Classes vs Structs in Action

Let’s do a basic performance comparison using a benchmark tool like BenchmarkDotNet to illustrate real differences between classes and structs.

[MemoryDiagnoser]
public class StructVsClassTest {

    public struct MyStruct {
        public int A, B;
    }

    public class MyClass {
        public int A, B;
    }

    [Benchmark]
    public void StructTest() {
        MyStruct[] items = new MyStruct[1000000];
        for (int i = 0; i < items.Length; i++) {
            items[i].A = i;
            items[i].B = i;
        }
    }

    [Benchmark]
    public void ClassTest() {
        MyClass[] items = new MyClass[1000000];
        for (int i = 0; i < items.Length; i++) {
            items[i] = new MyClass() { A = i, B = i };
        }
    }
}

**Results:** In typical cases, the struct version runs faster and consumes less memory — unless the struct is large or frequently boxed. So choose wisely based on the actual use case.

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🏁 Summary Table: Class vs Struct

Aspect	Class	Struct
Type	Reference	Value
Stored in	Heap	Stack
Copy Behavior	Shallow (reference)	Deep (by value)
Inheritance	Supported	Not Supported
Garbage Collected	Yes	No
Performance	Slower for small objects	Faster for small objects

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🎯 Final Recommendation: Class or Struct?

Here’s the quick takeaway:

• Use a struct when your type is small, immutable, and logically represents a value.
• Use a class when your object is complex, mutable, and needs identity or polymorphism.

Don't default to one over the other. Analyze your use case and choose accordingly. That’s how you’ll write clean, maintainable, and performant C# code.

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📌 Conclusion

Understanding the difference between classes and structs in C# isn’t just academic — it directly affects your application’s performance, memory usage, and code clarity. Structs offer better performance for small, immutable data types, while classes shine in complex, reference-based object models that require inheritance and polymorphism.

If you’re designing a lightweight type that’s short-lived and frequently created or passed around (e.g., Point, Color, DateTime), go with a struct. But when you need to build a richer object hierarchy, manage shared state, or implement OOP principles like polymorphism, a class is the way to go.

Always evaluate your specific scenario, performance needs, and readability goals before choosing between them. Making the right decision can lead to cleaner, faster, and more reliable applications in C#.

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❓ Frequently Asked Questions

1. Can a struct inherit from another struct in C#?

No. Structs in C# do not support inheritance. They can implement interfaces but cannot inherit from another struct or class.

2. Is it possible to make a struct nullable?

Yes. You can declare a nullable struct using the ? symbol, like Point?. This is commonly used when working with value types that may be absent or undefined.

3. Why does changing a class variable affect other references?

Classes are reference types. When you assign a class instance to another variable, both variables point to the same memory location. Changing one reflects in the other unless explicitly copied.

4. When should I avoid using structs?

Avoid structs when your type is large, mutable, or requires polymorphism. Misusing structs can lead to performance issues due to boxing and unnecessary value copying.

5. Can structs have methods and constructors?

Yes. Structs can have constructors (as long as all fields are initialized), methods, properties, and even implement interfaces. However, they can’t have a default parameterless constructor or destructor.

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