Question

Consider that I have an interface that contains the following property

interface IFoo
{
    Int32 Id { get; }
}

Now say I want to create an IMutableFoo interface. Logically I would think that the following was correct:

interface IMutableFoo: IFoo
{
    Int32 Id { set; }
}

My thought is that it would inherit Id and then in my child interface I’d make it settable. To my surprise, this did not work. Instead I get a warning letting me know that I’m in fact overriding the Id in IFoo with the Id in IMutableFoo. I tried changing { set; } to { get; set; } with the same results. How do I do this right? In Java, I would simply add a setId method. How do I do this in C#? Thanks!

Answer 1

Re-declaration of Id in the descendent interface indeed hides the one in the parent.

I use two different workarounds for this, entailing tradeoffs that I do not particularly like.

1 – Using an abstract or even a non-abstract class instead of a mutable interface.

interface IFoo {
    Int32 Id { get; }
}
abstract class MutableFoo: IFoo {
    public abstract Int32 Id {get; set;}
}

The biggest drawback is that the users of your library can no longer program to an interface.

2 – Using a method instead of a property.

interface IFoo {
    Int32 Id { get; }
}
interface IMutableFoo: IFoo {
    void SetId(Int32 value);
}

This is not ideal, because the setter is not idiomatic, and looks disconnected from the getter.