What "Init only setters" provides and what is the difference to the readonly one in C# [duplicate]

Question

I can define a class like below:

public class MyClass
{
  public int Id { get; }
  public MyClass(int id) => Id = id;
}

And I have to define the Id from the constructor and it will be read-only.

But if I want to use Init only setters in the C# 9.0, what does it and how can I use it?

public class MyClass
{
  public int Id { get; init; }
}

Answer 1

In a nutshell:

var obj = new MyClass
{
    Id = 42 // totally fine
};

obj.Id = 43; // not OK, we're not initializing

Trivial in this case and not much different to using a constructor parameter, but useful in some more complex scenarios where you don't want 200 constructor parameters, but you do want it to be outwardly immutable once constructed.