with expression + interfaces

[JanEggers]

right now im having a bunch of interfaces describing my model. I would like to convert the implementations into records.

so the interface gets init properties instead of readonly properties.

but how do I express that the object is withable in the interface?

[Joe4evr]

You can't right now.

For the next version of C#, the language team intends to add [Factory]: an attribute that indicates a method return is always a fresh instance, so it's okay to put an object initializer after such a call. If you then define a method named With that takes no parameters and returns the same object type, the compiler will use that pattern to allow with as a keyword on instances of that object.

public interface IMyModel
{
    [Factory]
    IMyModel With();
}

See here.

[333fred]

How we intend to add either factories or the general "withable" constraint is still very much undecided. I wouldn't say anything about what we intend beyond a simple we intend to support this.