Allow set to override and implement init

[YairHalberstadt]

The following scenarios should be legal:

public interface I
{
    public int P { get; init; }
}

public class C : I
{
    public int P { get; set; }
}

public class Base
{
    public abstract int P { get; init; }
}

public class Derived : Base
{
    public override int P { get; set; }
}

[liamlim]

I like this proposal and it solves all problems of #4030 that I have. Do I understand correctly that following code would be legal?

public class Base
{
    public abstract int P { get; init; }
}

public class Derived : Base
{
    public void SetP() => P = 42;

    public override int P { get; private set; }
}

var d = new Derived 
{
  P = 42
};
d.SetP();

[YairHalberstadt]

I don't think so - we would need a new feature where you could have something both init, and private set.

The problem is init is both wider than private set because it allows setting the property publicly in object initializer, and less wide, since it doesn't allows setting it at all outside object initializers.

[liamlim]

I imagined init property as setter with init accessibility modifier which is less accessible than private because it allows to assign only during creation of object. What is wrong about this implementation? I agree about less wide part but I don't agree about wider part. We cannot call object initializer more times to change assigned value thus I don't agree that the value is publicly available

[liamlim]

Compare following (I marked differences of both definitions with bold)

class A
{
  public A(int a)
  {
    A = a;
  }

  public A { get; private set; }
}

With this definition, we can assign to private setter using public constructor during construction of an object. This DOES NOT mean that the setter is publicly available because constructor can be called only once.

class A
{
  public A { get; init /* set */; }
}

With this definition, we can assign to init setter using public property during construction of an object. This DOES NOT mean that the setter is publicly available because initializer can be called only once.

[JeroenBos]

public class Base
{
    public abstract int P { get; init; }
}

Currently, class Base can assume that property P won't change after the initializer, which in a useful invariant.

This proposal would break that invariant.

[YairHalberstadt]

Currently, class Base can assume that property P won't change after the initializer, which in a useful invariant.

This proposal would break that invariant.

No it can't:

public class Base
{
    public abstract int P { get; init; }
}

public class Derived : Base
{
    public PBackingProperty { get; set; }
    public override int P { get => PBackingProperty; init => PBackingProperty = value; }
}