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

Answer 1

Make the constructor internal and house the factory in the same assembly.

public MyClass
{
    internal MyClass()
    {
    }
}

in same assembly

public MyClassGenerator
{
    public static CreateMyClass()
    {
        return new MyClass();
    }
}

If the factory can't be in the same assembly or this method doesn't work for you, look to Dan's answer