C# Academy · Lesson

Primary Constructors with Structs

Apply primary constructors to value types.

Lesson 3 of 413 steps

Primary Constructors with Structs is a free C# Academy lesson on CoddyKit. This is lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

Primary Constructors on Structs

Structs support primary constructors too: struct Point(int x, int y). They work much like on classes, declaring parameters in the header.

using System;

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

var p = new Point(3, 4);
Console.WriteLine(p.X + "," + p.Y); // 3,4

A Parameterless Constructor Still Exists

Structs always have an implicit parameterless constructor producing the default value. A primary constructor adds a parameterized one alongside it.

using System;

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

var zero = new Vec(); // default: x=0, y=0
Console.WriteLine(zero.X + "," + zero.Y); // 0,0

Exposing Parameters via Properties

As with classes, struct primary constructor parameters are not public properties automatically; expose them explicitly.

using System;

struct Size(int w, int h) {
    public int Width { get; } = w;
    public int Height { get; } = h;
}

var s = new Size(10, 20);
Console.WriteLine(s.Width * s.Height); // 200

Computed Members

Struct primary constructors pair nicely with computed, expression-bodied members.

using System;

struct Complex(double re, double im) {
    public double Magnitude => Math.Sqrt(re * re + im * im);
}

Console.WriteLine(new Complex(3, 4).Magnitude); // 5

Readonly Structs

A readonly struct with a primary constructor guarantees immutability. All members must not mutate state, which fits get-only properties perfectly.

using System;

readonly struct Money(decimal amount, string currency) {
    public decimal Amount { get; } = amount;
    public string Currency { get; } = currency;
    public string Display => Amount.ToString("0.00") + " " + Currency;
}

Console.WriteLine(new Money(5m, "USD").Display); // 5.00 USD

Chaining to the Primary Constructor

Additional struct constructors must call the primary one with : this(...), just like classes.

using System;

struct Square(int side) {
    public int Side => side;
    public int Area => side * side;
    public Square() : this(1) { } // explicit default side
}

Console.WriteLine(new Square(4).Area); // 16

Default vs Primary Constructor

Note that new Square() can use either the implicit zero-init or your declared parameterless constructor. If you define one that chains, it runs your logic.

using System;

struct Pct(double value) {
    public double Value { get; } = value;
    public Pct() : this(100) { }
}

Console.WriteLine(new Pct().Value); // 100

Value Semantics Remain

Structs still copy by value. A primary constructor does not change that: assigning a struct copies all its fields.

using System;

struct Counter(int n) {
    public int N { get; set; } = n;
}

var a = new Counter(1);
var b = a;       // copy
b.N = 99;
Console.WriteLine(a.N + " " + b.N); // 1 99

Initializing Mutable Fields

You can initialize settable properties or fields from parameters, though immutable structs are usually preferable.

using System;

struct Bag(int capacity) {
    public int Capacity { get; set; } = capacity;
    public int Used { get; set; } = 0;
}

var bag = new Bag(10);
bag.Used = 3;
Console.WriteLine(bag.Capacity - bag.Used); // 7

Deconstruction

Pair a primary constructor with a Deconstruct method to get tuple-like unpacking.

using System;

struct Point(int x, int y) {
    public int X => x;
    public int Y => y;
    public void Deconstruct(out int a, out int b) { a = x; b = y; }
}

var (px, py) = new Point(7, 8);
Console.WriteLine(px + "," + py); // 7,8

Putting It Together

A small immutable value type built with a readonly struct and a primary constructor.

using System;

readonly struct Temperature(double celsius) {
    public double Celsius { get; } = celsius;
    public double Fahrenheit => Celsius * 9 / 5 + 32;
}

var t = new Temperature(25);
Console.WriteLine(t.Fahrenheit); // 77

Quick Check

Test your grasp of primary constructors with structs.

Answer it in the interactive lesson

Recap

You learned primary constructors on structs.

struct P(int x, int y) declares parameters in the header.
The implicit parameterless constructor still exists.
Combine with readonly struct for immutable value types.
Extra constructors chain with : this(...).

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Apply primary constructors to value types. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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