Class instance creation restrictions

Question

I have many algorithm classes that implement the same interface; another "factory" class has the responsibility to instantiate the correct algorithm class through a config param, then call the start method on that instance.

I would like to restrict the the visibility of the constructor (or any other instance creation mechanism) of the algorithm classes to the factory class only.

How can I solve this? The only clean solution I can think about is to move those classes in a different .dll and change algorithm classes to private, but that's not what I want to do right now.

Answer 1

Maybe you will be satisfied by the following sample (yes, I know what reflection shouldn't be used normally...)

public class Algorithm
{
    private Algorithm()
    {
    }

    public void SomeMethod()
    {
    }
}

public static class Factory
{
    public static Algorithm Create()
    {
        var constructor = typeof(Algorithm).GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, null, new Type[0], null);
        return (Algorithm)constructor.Invoke(null);
    }
}

Here you can create instance of Algorithm via Factory.Create, not by new Algorithm.