readonly struct

[truegoodwill]

Since all fields of the OneOf struct are readonly, can we also mark the struct readonly using the keyword in its declaration?
I was surprised to discover this has not already happened and suspect you have a good reason. I'm looking to improve performance in method calls by passing the struct with the in keyword.

[mcintyre321]

I haven't really used readonly before so I'm unsure of pros/cons of adding it. Anyone able to provide more informations on risks/downsides? If there are none, I would happily accept a PR.

[kemsky]

@mcintyre321, you can read more here: https://devblogs.microsoft.com/premier-developer/avoiding-struct-and-readonly-reference-performance-pitfalls-with-errorprone-net/

tldr; if struct is not readonly then compiler must create defensive copies in some cases to maintain semantics, most notably when you have readonly field:

private readonly OneOf<MyStruct1, MyStruct2> _value = new MyStruct1();

In the example above _value is copied each time you call Match or any other method that is not marked as readonly to ensure that _value does not change.

[cremor]

The initial ask was completed by #129.
But the structs in OneOf.Types could also be made readonly.