
# Choosing Between Struct, Class, and Collections in ASP.NET Core: A Practical Decision Matrix

In a typical three-tier ASP.NET Core API (API, Service, Data/Proxy), choosing between `struct`, `class`, and different collection types can impact performance, maintainability, and correctness. Here’s a decision matrix with real-world C# examples for each scenario.

## How Much Data/Size: Struct vs Class?

**General Rule:**

- Use `struct` for small, immutable value types (typically ≤16 bytes, up to 2–4 fields, no reference-type fields).
- Use `class` for larger objects (more than 16 bytes, many fields, or containing reference types).

**Why?**

- Small structs are fast to copy and store inline (on the stack or inside arrays/objects).
- Large structs (over 16 bytes or 3–4 fields) become expensive to copy, so use `class` to avoid performance issues.

**Microsoft’s guideline:**

> "A struct should be less than 16 bytes and ideally immutable. If it’s larger, use a class instead." ([.NET docs](https://learn.microsoft.com/en-us/dotnet/standard/design-guidelines/struct))

## Decision Matrix

| Scenario                                    | Choice                       | Reasoning                                                                                 |
| ------------------------------------------- | ---------------------------- | ----------------------------------------------------------------------------------------- |
| Small value object (e.g., Money, Point)     | struct inside class          | Inline storage, zero extra allocations, fast, avoids heap allocation, value semantics     |
| Large value object (>16 bytes, many fields) | class inside class           | Copying large structs is expensive; class avoids unnecessary copying, reference semantics |
| Collection of small data                    | List<struct>                 | Compact, cache-friendly, no per-item heap allocation, efficient for value types           |
| Collection of large data                    | List<class>                  | Avoids copying large objects, allows sharing references, better for polymorphism          |
| Shared mutable state                        | class inside class           | Multiple references to same object, enables shared state and mutation                     |
| Immutable data (readonly)                   | readonly struct inside class | Best performance + safety, thread-safe, no accidental mutation, value semantics           |

---

## 1. Small Value Object (e.g., Money, Point)

**Use:** `struct` inside a class

```csharp
public struct Point
{
    public int X { get; }
    public int Y { get; }
    public Point(int x, int y) => (X, Y) = (x, y);
}

public class Shape
{
    public Point Center { get; set; }
}
```

**Why?**

- No heap allocation for `Point`.
- Value semantics (copy by value).

---

## 2. Large Value Object (>16 bytes, many fields)

**Use:** `class` inside a class

```csharp
public class Address
{
    public string Street { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    public string Zip { get; set; }
    public string Country { get; set; }
}

public class Customer
{
    public Address ShippingAddress { get; set; }
}
```

**Why?**

- Avoids expensive copying of large structs.
- Reference semantics.

---

## 3. Collection of Small Data

**Use:** `List<struct>`

```csharp
public struct Pixel
{
    public byte R, G, B;
    public Pixel(byte r, byte g, byte b) => (R, G, B) = (r, g, b);
}

public class Image
{
    public List<Pixel> Pixels { get; set; } = new List<Pixel>();
}
```

**Why?**

- Compact, cache-friendly storage.
- No per-item heap allocation.

---

## 4. Collection of Large Data

**Use:** `List<class>`

```csharp
public class Document
{
    public string Title { get; set; }
    public string Content { get; set; }
}

public class Library
{
    public List<Document> Documents { get; set; } = new List<Document>();
}
```

**Why?**

- Avoids copying large objects.
- Allows sharing and polymorphism.

---

## 5. Shared Mutable State

**Use:** `class` inside a class

```csharp
public class Counter
{
    public int Value { get; set; }
}

public class Game
{
    public Counter Score { get; set; } = new Counter();
}

// Multiple references can share the same Counter instance
```

**Why?**

- Multiple references to the same object.
- Enables shared, mutable state.

---

## 6. Immutable Data (readonly)

**Use:** `readonly struct` inside a class

```csharp
public readonly struct Temperature
{
    public double Celsius { get; }
    public Temperature(double celsius) => Celsius = celsius;
}

public class WeatherReport
{
    public Temperature Current { get; }
    public WeatherReport(Temperature current) => Current = current;
}
```

**Why?**

- Thread-safe, no accidental mutation.
- Best performance for small, immutable data.

---

## Summary Table

| Scenario                 | Use This                     |
| ------------------------ | ---------------------------- |
| Small value object       | struct inside class          |
| Large value object       | class inside class           |
| Collection of small data | List<struct>                 |
| Collection of large data | List<class>                  |
| Shared mutable state     | class inside class           |
| Immutable data           | readonly struct inside class |

**Tip:**

- Use `struct` for small, immutable, value-like data (≤16 bytes, no inheritance, no shared state).
- Use `class` for large objects, shared/mutable state, or when reference semantics are needed.
- Use `readonly struct` for immutable value types to prevent accidental mutation.
- Use collections of structs for small, frequently accessed data; use collections of classes for large or polymorphic data.

---

**Have questions or want to see more advanced scenarios? Leave a comment below!**


A practical guide for ASP.NET Core developers on when to use struct, class, and collections, with real-world C# examples for each scenario.

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Choosing Between Struct, Class, and Collections in ASP.NET Core: A Practical Decision Matrix

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