How to construct an interface over types which contain different signatures, but with params all corresponding to the same type

I have several car factories:

HondaFactory, MercedesFactory, ToyotaFactory

All of these factories have a Create() method:

Honda Create(HondaParts parts);
Mercedes Create(MercedesParts parts);
Toyota Create(ToyotaParts parts);

All return types implement ICar, and all parts implement ICarParts.

I have a class above all this which, when given a factory type and an ICarParts, wants to simply call Create() on the factory without an if/else/if/else statement on the type.

I went to create an ICarFactory and give it an interface of:

ICar Create(ICarParts parts)

But then I got stuck, as of course each factory has a different signature, even though all of the parameters comply with the ICarParts type. Is there any way to achieve what I’m after?

>Solution :

Use generics

abstract class Factory<TParts> where TParts : ICarParts 
{
    public abstract ICar Create(TParts parts);
}

class HondaFactory : Factory<HondaParts>
{
    public override ICar Create(HondaParts parts)
    {
        ...
    }
}

... other factories

But as always with generics there are limits. E.g., you cannot create a List<Factory<?>> containing different types of factories. The different types of factories are not assignment compatible.

Since C# 9.0 (and .NET 5.0 runtime, I think) you can use covariant return types :

    public override Honda Create(HondaParts parts)
    {
        ...
    }

assuming public class Honda : ICar.