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readonly struct

Context: readonly struct indicates that the struct is immutable. All instance fields must be readonly, and the compiler enforces immutability.

public readonly struct Vector
{
    public double X { get; }
    public double Y { get; }
    public Vector(double x, double y) => (X, Y) = (x, y);
    public double Length => Math.Sqrt(X * X + Y * Y);
}

Benefits

Clear intent: this struct cannot be modified after creation
Performance: compiler can avoid defensive copies
Thread‑safe by design

`readonly` members

Even if the struct is not readonly, you can mark individual methods as readonly to prevent modification.

public struct Point
{
    public int X, Y;
    public readonly double DistanceFromOrigin() => Math.Sqrt(X * X + Y * Y);
}

Real-world usage example

Immutable settings: Use readonly struct for configuration values that never change (e.g., AppConstants with double Pi, int MaxRetries).

Example: In System.Text.Json, serialization options can be readonly structs for performance.