Proposal: "with" expressions for mutable objects

[alrz]

This has been concluded in #5365 but I thought it deserves a separate ticket. Also it can be considered as an extension to #5172 for using with expressions on other types.

It basically provides a With...End With-like syntax for C# but as an expression.

var car = with carFactory.CreateCar() {
    Brand = "Ferrari",
    Model = "Enzo"
};

->

var car = carFactory.CreateCar();
car.Brand = "Ferrari";
car.Model = "Enzo";

However, you can ignore the returned value if you don't need it e.g. after with car { ... }; you can continue to use the existing variable.

[orthoxerox]

How will you reconcile the difference in semantics between withing an immutable (create a copy and modify it) and a mutable (mutate the object passed to the expression) object?

[alrz]

@orthoxerox with mutableCar { ... } vs existing syntax immutableCar with { ... }. The former implies "with mutableCar in the context" and the latter implies "new immutableCar with following properties".

[orthoxerox]

@alrz wouldn't this still be too confusing?

[alrz]

@orthoxerox Programmers should be familiar with this kind of "confusing"! For example const in C++ is pretty confusing:

const char* pstring = "";
char* const pstring = "";
const char* const pstring = "";

These are all different.

[MgSam]

So because a completely separate language has a confusing design for a completely separate feature means it's ok for C# to add confusing features?

[aluanhaddad]

Factory initializers are a good idea. Using the same keyword that is specifically intended for use in immutable context for a mutability oriented feature is not

[ashmind]

Duplicate of #133?

[alrz]

@ashmind the use case is partially the same but you didn't mentioned a practical syntax, it's the same as with for record types which as @orthoxerox mentioned these should be different to some degree.
However, the intention for this proposal is not limited to factory initialization, it's more of a With...End With syntax that actually covers the factory initialization too (that's why I didn't see your proposal in the first place) and followed by #5365 this shouldn't be limited to just properties.

[alrz]

@aluanhaddad you're saying that this should use another keyword? my proposed syntax is based on the fact that these two basically do the same job with a different implementation. And since I'm against of totally immutable records (well, not algebraic types, just records; see #206 comment) this would make sense that you actually decide how you want to do this when you are doing it — like C++ classes; you decide where you want to put your variable (stack or heap) right when you are declaring it, not based on type.

[alrz]

As an alternative syntax this could be done with no keyword at all e.g. scoping braces with a leading expression var car = carFactory.CreateCar() { ... };.

[gafter]

@alrz This appears to be a dup of #133 with a different syntax.
#5365 does not appear to have much to do with this proposal, even though it is a separate proposal for a similar construct.