make sure object only created by factory (C#)

Question

How do I make sure that a certain class is only instantiated by a factory and not by calling new directly?

EDIT: I need the factory to be a separate class (for dependency injection purposes) so I can't make it a static method of the class to be instantiated, and so I can't make new private.

Answer 1

If, for some reason, you need the factory and the constructed class to be in separate assemblies (which means simply using internal won't work), and you can ensure that your factory gets a chance to run first, you can do this:

// In factory assembly:

public class Factory
{
    public Factory()
    {
        token = new object();
        MyClass.StoreCreateToken(token);
    }

    public MyClass Create()
    {
        return new MyClass(token);
    }

    private object token;
}

// In other assembly:

public class MyClass
{
    public static void StoreCreateToken(object token)
    {
        if (token != null) throw new InvalidOperationException(
            "Only one factory can create MyClass.");

        this.token = token;
    }

    public MyClass(object token)
    {
        if (this.token != token) throw new InvalidOperationException(
            "Need an appropriate token to create MyClass.");
    }

    private static object token;
}

Yes, it's cumbersome and awkward. But there may be weird situations where this is actually a good solution.

Answer 2

It will always be created by calling new somewhere, but if you only want that to happen in your factory class, you can set all the constructors to Internal (or Private, and use a Public Static factory method on the same class).