C#•2y ago

❔ How to return a generic interface whose only parameter is determined on runtime?

So, I've got this code, it doesn't compile but it clarifies what I want to do:

    private static IProblem<T>? GetProblem<T>(int day, T type)
    {
        try
        {
            var instance = Activator.CreateInstance(Problems[day]);
            return (IProblem<T>?)instance;
        }
        catch (Exception e)
        {
            Console.Error.WriteLine(e);
            return null;
        }
    }

    private static IProblem<T>? GetProblem<T>(int day, T type)
    {
        try
        {
            var instance = Activator.CreateInstance(Problems[day]);
            return (IProblem<T>?)instance;
        }
        catch (Exception e)
        {
            Console.Error.WriteLine(e);
            return null;
        }
    }

The variable instance is of type object, I want it to be of IProblem<T> where T is a generic parameter of an interface that Problems[day] implements. I've been butting my head with this code for quite a while, but I can't find something that works. How could I accomplish this with generics? Or do I have to go dynamic?

17 Replies

Tinefol•2y ago

Yoiu can't use new, need to specifically use Activator class?

Aaron•2y ago

what is the type of Problems

E-slackerOP•2y ago

I can't use new with a Assembly that I get on runtime, can I? What I want to do, basically, is to get all classes of the project that implement IProblem<> and call some of their methods Type[] of different classes all implementing IProblem<>

Aaron•2y ago

what is the error you get with this block

E-slackerOP•2y ago

InvalidCastException System.InvalidCastException: Unable to cast object of type 'Problems.Day1.Day1' to type 'Utils.IProblem`1[System.Type]

Aaron•2y ago

oh, it compiles, just doesn't run

E-slackerOP•2y ago

Oh yeah my bad Day1 implements IProblem<int>

Aaron•2y ago

Day1 doesn't implement IProblem<Type> you gave it Type for T

E-slackerOP•2y ago

Oh Given a Type object, how can i get its actual type?

Aaron•2y ago

to use as a generic? you can't, really is IProblem your interface?

E-slackerOP•2y ago

Yes Should I just use dynamic?

Aaron•2y ago

could just get rid of the generic on IProblem, just using object instead dynamic would also probably work? no reason to use it if you don't have to

E-slackerOP•2y ago

oh well, there's absolutely no way to use generics in this case?

Aaron•2y ago

not if you want to return it as an actual instance of IProblem

E-slackerOP•2y ago

I see, thanks a lot for your time

cap5lut•2y ago

i didnt touch it myself, but is this what u r looking for? https://learn.microsoft.com/en-us/dotnet/api/system.type.makegenerictype?view=net-7.0

Type.MakeGenericType(Type[]) Method (System)

Substitutes the elements of an array of types for the type parameters of the current generic type definition and returns a Type object representing the resulting constructed type.

Accord•17mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ How to return a generic interface whose only parameter is determined on runtime?