Design Guidelines for C# Structs (C Sharp)

Structs in C# can do most of what classes can do, but there are important differences that affect how you should design and use them. Understanding these guidelines will help you write more efficient, maintainable code and avoid common pitfalls.

The key question isn't whether structs are better or worse than classes—it's knowing when each is the right tool for the job. Let's start by understanding when you should choose a struct over a class.

When to Use Structs

You should consider using a struct when your type meets these criteria:

• Small Size: Instances are relatively small (typically 16 bytes or less)
• Short Lifetime: The instance lifetime is very short
• Embedded Usage: You'll embed instances into other objects or collections
• Single Value: Logically represents a single value (like Point, Color, or Rectangle)

Common examples of good struct candidates include coordinates (X, Y), colors (R, G, B, A), dates and times, and small mathematical types. If your type doesn't fit these criteria, you're probably better off using a class.

Essential Design Guidelines

Represent a Single Value

Logically, your struct should represent one value. Think of types like int or decimal—they represent single values. Your struct should follow the same principle. A Point struct with X and Y coordinates represents one concept: a location.

Keep It Small

Keep your struct size under 16 bytes. Larger structs lose the performance benefits of being value types. When structs get too big, copying them becomes expensive, and you negate the advantage of stack allocation.

Make It Immutable

Design your struct to be immutable whenever possible. This means once you create it, you can't change its values. Immutable structs are safer, more predictable, and work better in multithreaded scenarios.

No Inheritance

Structs cannot inherit from other structs or classes, and they cannot be base types. This is a fundamental limitation. If you need inheritance, you must use a class instead.

Cannot Be Static

You cannot declare a struct as static. If you need a static type, use a static class instead.

Understanding Value Type Behavior

When you pass a struct to a method, it's passed by value—meaning the method gets a copy, not a reference to the original. This has important implications:

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

public void ModifyPoint(Point p)
{
    p.X = 100; // Only modifies the local copy
}

// Usage
Point myPoint = new Point(10, 20);
ModifyPoint(myPoint);
Console.WriteLine(myPoint.X); // Still 10, not 100!

The original myPoint remains unchanged because ModifyPoint received a copy. If you need to modify the original, pass the struct by reference using the ref keyword.

Boxing and Unboxing Performance

Structs are value types, which means allocating and deallocating them is faster than reference types. However, assigning a struct to an object variable requires boxing, and extracting it back requires unboxing. This creates performance overhead.

Point p = new Point(5, 10);

// Boxing - struct copied to heap
object obj = p;

// Unboxing - copied back from heap
Point p2 = (Point)obj;

Only use structs when you expect minimal boxing and unboxing. If your struct will frequently be cast to interfaces or object, the boxing overhead might make a class a better choice.

Constructor Restrictions

You cannot explicitly define a parameterless constructor for a struct. C# doesn't allow it because the runtime provides one automatically that initializes all fields to their default values:

public struct SampleStruct
{
    int member1;
    
    // This is allowed - constructor with parameters
    public SampleStruct(int data)
    {
        member1 = data;
    }
    
    // ERROR: Structs cannot contain explicit parameterless constructors
    public SampleStruct()
    {
        Console.WriteLine("Default Constructor");
    }
}

The compiler will give you an error: "Structs cannot contain explicit parameterless constructors." This restriction ensures that all struct instances have predictable default values.

Field Initialization Rules

You cannot directly initialize struct fields in their declarations. This is different from classes:

// ERROR: Cannot have instance field initializers in structs
public struct SampleStruct
{
    int member1 = 10;   // Not allowed!
    int member2 = 134;  // Not allowed!
}

// CORRECT: Initialize in constructor
public struct SampleStruct
{
    int member1;
    int member2;
    
    public SampleStruct(int m1, int m2)
    {
        member1 = m1;
        member2 = m2;
    }
}

While classes allow field initialization at declaration, structs require you to initialize fields in constructors. This ensures proper initialization order and consistent behavior.

Handling Default Values

Always check that struct fields are in a valid state before using them. Reference type fields default to null, which can cause runtime exceptions if you're not careful:

public struct SampleStruct
{
    string member;
    
    public SampleStruct(string data)
    {
        member = data;
    }
    
    public bool CompareString(string data)
    {
        // Always check both parameters AND member fields
        if (data != null && data.Length > 0 && 
            member != null && member.Length > 0)
        {
            return member.Equals(data);
        }
        return false;
    }
}

// Dangerous usage
SampleStruct obj1 = new SampleStruct(); // member is null!
bool isEqual = obj1.CompareString("Sample"); // Could crash without null checks

Even though you have a constructor that sets the member, someone can still create the struct using the default parameterless constructor. Your methods must handle fields being in their default state.

Implementing IEquatable

For structs, implement IEquatable<T> to provide efficient equality comparisons. This interface eliminates boxing and reflection overhead compared to using the default Equals method:

public struct Point : IEquatable
{
    public int X { get; }
    public int Y { get; }
    
    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }
    
    // Efficient equality comparison without boxing
    public bool Equals(Point other)
    {
        return X == other.X && Y == other.Y;
    }
    
    // Override Object.Equals for consistency
    public override bool Equals(object obj)
    {
        return obj is Point other && Equals(other);
    }
    
    // Also override GetHashCode when overriding Equals
    public override int GetHashCode()
    {
        return HashCode.Combine(X, Y);
    }
}

Summary

Structs are powerful when used correctly, but they come with specific rules and limitations. Keep them small, immutable, and use them to represent single values. Understand the performance implications of boxing and unboxing, and always account for default values when working with struct fields.

Following these design guidelines will help you write efficient, maintainable code that leverages structs where they shine while avoiding common pitfalls that can lead to bugs and performance issues.