In C#, how do I add functionality to an already constructed class?

Question

Assume I already have a duck. Given that the Duck is readonly and C# is GC'd, is there any way to drop a duck into a mallard with inheritance without hitting a copy constructor?

public class Duck
{
    public readonly int wings;
    public Duck(int w)
    {
        wings = w;
    }
}

public class Mallard : Duck
{
    public Mallard(Duck a) : base(a.wings)
    {
    }
}

You can't "promote" an instance of one type to an instance of a derived type. `Duck` probably should be abstract, anyway. — madreflection, Oct 21 '19 at 19:33
"without hitting a copy constructor" You've only defined one constructor for the `Mallard` class. The only way to create a Mallard object is to run that constructor. — gunr2171, Oct 21 '19 at 19:33
make the parent data for mallard be a duck, but not create a new duck. — Carbon, Oct 21 '19 at 19:33
`var duck = new Duck(3); duck = new Mallard(duck);` You really ought to specify which side the extra wing is on, though. That'll affect navigation. Also there's a chicken/egg issue. — 15ee8f99-57ff-4f92-890c-b56153, Oct 21 '19 at 19:34
"parent data" suggests composition. Now you're talking about the decorator pattern. But you can't avoid creating a new instance of *something*. — madreflection, Oct 21 '19 at 19:37
I assume under the hood the base object is just a ref though? Can you point it to the other instantiated object given that the base object in this case can never change? — Carbon, Oct 21 '19 at 19:38
what does "not create a new duck" mean? do you need to modify a certain instance of a Duck object to behave like a Mallard? https://stackoverflow.com/questions/7299097/dynamically-replace-the-contents-of-a-c-sharp-method — Tiberiu Maran, Oct 21 '19 at 19:38
Are you trying to translate `static_cast(duck)` (C++) to C#? — madreflection, Oct 21 '19 at 19:40
It would be unsafe to assume that the memory layout of `Mallard` is the same as `Duck`, even if *you* don't add any fields or virtual overrides to `Mallard`, so it's not possible. — madreflection, Oct 21 '19 at 19:42
@madreflection note that in C++ that's undefined behavior if `Duck` is not actually a `Mallard`, and even if you don't hit anything sensitive, the polymorphic behavior of the `Mallard` will still be as a `Duck`, because you're not changing the vtable of the instance — Tiberiu Maran, Oct 21 '19 at 19:43
@artcorpse: I'm quite aware of that. It doesn't change the fact that *the OP* is trying to do it. — madreflection, Oct 21 '19 at 19:44
@Carbon It might help to understand why you need such a static_cast, as it doesn't seem to accomplish much. If you need to operate with methods from type `Mallard` on a `Duck` something like this might work: https://blog.tchatzigiannakis.com/changing-an-objects-type-at-runtime-in-c-sharp/. — Tiberiu Maran, Oct 21 '19 at 19:56
More importantly, will your design also need to convert the river to whiskey? — 15ee8f99-57ff-4f92-890c-b56153, Oct 21 '19 at 20:04

Kyle W · Accepted Answer · 2019-10-21T19:39:32.130

A Duck is not a Mallard, and in C# there's no way to make it be one. You're going to have to construct a new Mallard in some way, whether that's by a copy constructor or other method.

In some cases, you may be able to go with composition over inheritance (although I don't think it models your domain in this example).

public class Mallard
{
   public readonly Duck duck;
   public Mallard(Duck d)
   {
      duck = d;
   }
}

If you're just looking to add new functionality to a Duck, but can't change it, you can use extension methods.

public static class DuckExtensions
{
   public IEnumerable<Wing> CookWings(this Duck duck) { ... }
}

In C#, how do I add functionality to an already constructed class?

1 Answers1