C# Academy · Lesson

Immutability with init & with

Use init-only setters to create immutable objects and with-expressions to produce non-destructively modified copies.

What Is Immutability?

An immutable object cannot be changed after creation. Immutability prevents accidental mutation, makes objects thread-safe by default, and simplifies reasoning about code. C# provides init setters and with expressions to make immutability ergonomic.

init-Only Setters

An init accessor allows a property to be set during object initialization (constructor or object initializer) but never after. It's the immutable counterpart to set.

public class Point
{
    public double X { get; init; }
    public double Y { get; init; }
}

var p = new Point { X = 3.0, Y = 4.0 }; // OK — init phase
p.X = 5.0; // COMPILE ERROR — cannot set after init

// All of these are valid init-phase assignments:
var p2 = new Point(X: 1.0, Y: 2.0);
var p3 = new Point { X = 0, Y = 0 };

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