Question

I have an interface ISnack which when implemented by a class, it should have a default parameterless constructor. Basically this:

public interface ISnack<T> where T : new()
{

}

I use <T> where T : new() just to enforce the parameterless constructor.

I would then implement the interface this way:

public class Cutlet : ISnack<Cutlet>
{

}

This works and it simply ensures Cutlet class has a parameterless constructor.

Now I have an abstract base class Kitchen:

public abstract class Kitchen<T> where T : ISnack
{

}

The requirement is that Kitchen should have constraint where T should be an ISnack. But this wont work because there exists no ISnack, but only ISnack<T>.

If I tried this

public abstract class Kitchen<T> where T : ISnack<T>
{

}

it wouldn’t compile ('T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'ISnack<T>') and also wouldn’t make sense in my context.

If I could force ISnacks to have a parameterless constructor without constraining by a T type parameter, then T in Kitchen<T> could easily be an ISnack. How to go about it?

Answer 1

You can’t unless you add the constraint; generic constraints are cumulative, so to make the compiler happy you would have to have:

public abstract class Kitchen<T> where T : ISnack<T>, new()

If that is fine, then do that. If it isn’t fine, then you’ll have to remove the : new from the original, and make do without it. This isn’t as bad as it sounds, but it means you push validation down to execution rather than compilation. But: Activator.CreateInstance<T>() still does what you would need, anyway – even without the new() constraint. So you can replace:

T newObj = new T(); // validated by the compiler

with:

T newObj = Activator.CreateInstance<T>(); // not validated until executed

A handy trick when removing constraints can be: add a unit/integration test that finds the candidate types via reflection, and validate the missing constraint as part of your test suite.