readonly struct Should not contain reference type

[MohamadAlallan-Pi]

Reference types in readonly struct breaks the design of immutable struct.

This code works pretty good as the readonly struct has value types only

static void Main(string[] args)
    {
        var b = new Bar { Value = "bar before foo" };
        var f = new Foo(b);
        Console.WriteLine(f.Value);
        b.Value = "bar after foo";

        f.Bar.Value = "bar from f.Bar";

        Console.WriteLine(f.Value);
    }
    readonly struct Foo
    {
        public string Value => Bar.Value;
        public Bar Bar { get; }
        public Foo(Bar b) => Bar = b;
    }
    struct Bar
    {
        public string Value { get; set; }
    }

but once you change Bar to a Reference Type: class Bar, things will break, you will have two issues

• The compiler error at line f.Bar.Value = "bar from f.Bar"; will disappear allowing the code to compile
• The f.Value is not readonly anymore as it can be manipulated by any reference to Bar (unlike when Bar was copied by value so other reference have no effect)

Having that, the compiler should prevent reference types from being used in readonly struct

[CyrusNajmabadi]

Read-only structs are not immutable, they are read-only. This has been the design of read-only in the language since 1.0. Changing this would break the world.

If you want to have this restriction in your own code, you can write an analyzer for that purpose.