[Proposal]: allow copying using "with" keyword with regular classes

[Aziz-Dhiab]

Summary

Currently, the with keyword in C# is restricted to records, allowing for non-destructive mutation by creating a shallow copy with modified properties. This proposal suggests extending with to regular classes, making object copying and modifications more concise and expressive using already existing syntax.

class User { ... }

User user2 = user1 with { Age = 30 };

Motivation

Improved Readability & Maintainability: The with syntax is more concise and readable than using a clone method and changing some property or using some workaround like this example

public User With(string? name = null, int? age = null) =>
       new User(name ?? Name, age ?? Age);

var user1 = new User("Bob", 25);
var user2 = user1.With(age: 30);

Records are great but there are a lot of situation where the user doesn't want value semantics on a class but copying using with keyword would greatly benefit in copying scenarios.

Drawbacks

Potential confusion: maybe only allow this if the class have a regular copy constructor.

[HaloFour]

See: #8876

I believe the biggest problem around adopting this for normal classes is enforcing the correct pattern for cloning the instance. A record emits a special "unspeakable" clone method, which is virtual and must be implemented by every type that derives from that record. The compiler would need to enforce a similar pattern for non-records. This method can't be a copy constructor because the compiler would not know the actual type of the instance at compile time, thus would call the constructor on the base type resulting in decapitation:

Person person = new Student() { ... };
Person clone = person with { ... };

Debug.Assert(clone is Student); // oops, the clone is not a Student since the Person constructor was called