With-expression support for copyable classes

[bernd5]

I have seen that there are proposals to extend the with-expression introduced with records to structs - but are there plans to allow with expressions for regular classes, too?

Of course such a class would need a parameterless Copy method and the correct return type.

[ufcpp]

see https://github.com/dotnet/csharplang/blob/a3d67d141501101af9dbb9e8192ca8b4c717e316/meetings/2020/LDM-2020-04-20.md

We'll consider broader Factory scenarios later.

[bernd5]

The extension to factory methods is a nice idea - but in my eyes it should be extended to copyable classes which have a parameterless method named "Copy" and a matching return type or at least for classes which implement ICloneable, too.

[ufcpp]

var x = new X { Value = 1 };
var y = x with { Value = 2 }; // error with C# 9.0 but ok with this proposal

Console.WriteLine(x.Value); // 2?

class X : ICloneable
{
    public int Value { get; init; }

    // ICloneable interface has no guarantee of creating new instance
    public object Clone() => this;
}

[bernd5]

Then you violate the ICloneable contract. I mean you could for example overload operator+ and implement there some substraction - this is crazy but no reason to disallow operator overloading.

But I understand such an implementation is intended as "Clone-optimization" which is perfectly legal as long as X and all of its child classes are immutable.