Question

I’ve created a class that takes in an object as a parameter by ref. When it takes this object in it’s construct it is null. Why does it stay null (uninitialized)?

// example code, not real
class Test{
    object parent;

    public Test (ref object _parent)
    {
        parent = _parent;
    }
 }

 object n = null;
 Test t = new Test (ref n);
 n = new Something ();

When I tried it with a different class (http://pastebin.com/PpbKEufr), it stayed null

Answer 1

Yes, parent will stay null regardless of what you eventually assign to the variable n. The ref keyword will pass the actual reference to n (as opposed to a copy of it) into your method, and is typically used if your method was intended to assign a new object instance to the parameter.

Inside your method, you are making a copy of the reference that n points to, in this case null. When the method completes both n and parent point to null. When later you assign to n, you haven’t changed the object to which n and parent point, rather you have asked n to point to a new location, leaving parent still pointing to null.

If you were to create an instance of an object and assign it to n, then parent would point to the same object and any changes to that object’s properties would be visible to your class. The same would be true if you did not pass the parameter as ref, as the reference copy also would point to the same object instance.

Hope that helps.