Structs, records and value vs reference types

Every C# type is either a value type or a reference type. You have met both already: int and double are value types; string, arrays and classes are reference types. This lesson shows how to create your own value types (structs) and the modern records.

1struct Point { public int X, Y; public Point(int x, int y) { X = x; Y = y; } }2 3var p1 = new Point(1, 2);4var p2 = p1;5p2.X = 10;6Console.WriteLine(p1.X);7Console.WriteLine(new Point(3, 4).Equals(new Point(3, 4)));8// new Point(3, 4) == new Point(3, 4)  → error CS0019

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Value types vs reference types

Structs

1public struct Point32{3    public double X { get; }4    public double Y { get; }5    public double Z { get; }6 7    public Point3(double x, double y, double z) { X = x; Y = y; Z = z; }8 9    public double DistanceTo(Point3 o) =>10        Math.Sqrt((X - o.X) * (X - o.X) + (Y - o.Y) * (Y - o.Y) + (Z - o.Z) * (Z - o.Z));11 12    public override string ToString() => $"({X}, {Y}, {Z})";13}14 15var a = new Point3(0, 0, 0);16var b = new Point3(3, 4, 12);17Console.WriteLine($"{a} → {b}: {a.DistanceTo(b)}");

(0, 0, 0) → (3, 4, 12): 13

Use a struct for small, simple values that are naturally copied: points, vectors, colours, coordinates, money amounts. 3D and BIM code uses structs everywhere for points and vectors, because millions of them can be stored in arrays without creating millions of heap objects.

• Keep structs small (roughly 16 bytes or less is the usual guideline) and preferably immutable (readonly struct).
• Structs can’t inherit from other structs or classes, but they can implement interfaces.
• Use a class when the data is large, needs identity (“this exact beam”), or must be shared and changed in one place.

Records: data classes in one line

1public record Section(string Name, double DepthMm, double KgPerMetre);2 3var s1 = new Section("IPE300", 300, 42.2);4var s2 = new Section("IPE300", 300, 42.2);5 6Console.WriteLine(s1);             // readable ToString for free7Console.WriteLine(s1 == s2);       // value equality: same data → equal8var heavier = s1 with { KgPerMetre = 49.1 };   // copy with one change9Console.WriteLine(heavier.KgPerMetre);

Section { Name = IPE300, DepthMm = 300, KgPerMetre = 42.2 }
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49.1

That single line generates a constructor, read-only (init) properties, Equals/== based on the values, GetHashCode, a nice ToString and deconstruction. Records are ideal for data that is passed around and compared: DTOs, settings, messages, results.

Choosing

1public record Money(decimal Amount, string Currency);2 3static Money Add(Money a, Money b)4{5    if (a.Currency != b.Currency)6        throw new InvalidOperationException("Currencies differ.");7    return a with { Amount = a.Amount + b.Amount };8}9 10var total = Add(new Money(1500, "INR"), new Money(750, "INR"));11Console.WriteLine(total);12Console.WriteLine(total == new Money(2250, "INR"));

Money { Amount = 2250, Currency = INR }
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