gistfile1.cs

public class SomethingGeneric<T> : SomethingNonGeneric
{
    public void DoSomething(T parameter);
}

public interface SomethingNonGeneric { }

public class SomethingStorage
{

    private List<SomethingNonGeneric> somethings = new List<SomethingNonGeneric>();
    
    public void AddSomething(SomethingNonGeneric something)
    {
        somethings.Add(something);
    }
    
    public Something<T> GetSomething<T>(int index)
    {
        return (Something<T>)somethings[index];
    }

}