C# 7.3 readonly struct type “this” reference

Question

readonly struct type

The this reference, instead of a copy of the receiver, is always an in parameter passed by reference to the member method. This optimization saves more copying when you use a readonly struct .

What exactly does this mean? Does normal struct always make a copy when using "this" in the constructor?

Answer 1

When you pass a struct as a parameter into a function, it makes a copy of the entire struct and gives it to the function.

Because you're using a readonly struct , you're essentially promising that it will never change.

When you use the (in) modifier on a function you pass it to, you're basically promising that you won't change the struct at all. So, it's safe to pass in as a reference (a pointer, basically), instead of copying the entire thing defensively.