Immutable/readonly structs (e.g. NSRange)

[Therzok]

This is not really a bug, but more of an open discussion about how some things are bound.

Take for example, NSRange: https://github.com/xamarin/xamarin-macios/blob/master/src/Foundation/NSRange.cs#L27

The struct is guidelines are for the struct to be immutable. (https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/struct)

Cocoa API doesn't seem to let it be mutated either: https://developer.apple.com/documentation/foundation/nsrange?language=objc

In case of structs, when a struct is in a readonly field/property or is passed as arguments to in parameters, the compiler generates a defensive copy.

This happens because this is mutable by default.

C# 7.2 has some nice-ness that allows for structs to be marked as readonly, thus not allowing the this variable to be set, but that requires the property to be fully readonly.

Why was NSRange designed mutable? Might be the case for most structs bound

[chamons]

We can't change the API of NSRange to be immutable until XAMCORE_4_0 I believe.

[spouliot]

Why was NSRange designed mutable?

@Therzok Not sure, that was done long ago (in Novell days) and not even in our git history. My guess is that the (then current) documentation and header files did not make that fact clear. E.g. headers used to include fields (not all of them being clear if they were private or not).

If it was to bound today it would be immutable but, right now, it must remain mutable since it would mean a breaking change.

[chamons]

I added it on the XAMCORE_4_0 audit list in case we ever wanted to change it, but since we're unlikely to break all platforms at once it'd be a bit of a flag day change.