Champion "Readonly ref" (C# 7.2)

[MadsTorgersen]

• Proposal added: proposals/csharp-7.2/readonly-ref.md
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed

[alrz]

I'm not quite clear why this has to be a separate feature from readonly locals and paramerters?

[jaredpar]

@alrz they are very different features.

• readonly ref: restricts the ability to mutate the target of a ref variable.
• readoonly locals: restricts the ability to retarget a variable to a new location.

These features do have some overlap with st. t values. But overall are different features.

[alrz]

So potentially, we could have readonly in parameters at some point?

[jaredpar]

@alrz

Possibly but that's a bit of a redundant notation. All ref identifiers are unassignable by default. Hence adding readonly is just making it more explicit for no real gain.

[alrz]

I'm under the impression that you are distinguishing something like const char* and char* const, does that analogy work here?

[gafter]

We might support reassignment of ref variables in the future. In order for that to be possible, we will need to distinguish between ref readonly (a ref to something you can't write), readonly ref (a ref that you can't reassign), and readonly ref readonly (a ref that you can't reassign to something you can't write).

[Thaina]

readonly ref readonly (a ref that you can't reassign to something you can't write).

Should just use const. const local and const parameter

for ref readonly and readonly ref. I think there are sealed,fixed and lock. 3 keywords that currently cannot be local or param modifier. We should reuse these

[ufcpp]

Can in parameters be contravariant?

interface ISetter<in T>
{
    void SetValue(in T value);
}

[jaredpar]

@ufcpp

Can in parameters be contravariant?

No. The CLI only truly recognizes ref parameters. The C# notions of in and out are a construct specific to C#. There is no underlying runtime support for enforcing these notions and hence it can't be a part of the core CLI type system.

[Athari]

and readonly ref readonly

And here I thought that C++-style of consts will never be considered for C#... Are ~~~const~~~ readonly functions also something being considered?

[BreyerW]

Could this feature enable ref readonly property with both setter and getter (getter being ref readonly returning while setter being normal method)? Main use case is to be able to reliably call OnValueChanged events and the like while still being able to benefit from ref performance gain on struct