Properties - by value or reference?

Question

I've got the following public property which exposes an Arraylist:

public ArrayList SpillageRiskDescriptions
        {
            get
            {
                return _SpillageRiskDescriptions;
            }
            set
            {
                _SpillageRiskDescriptions = value;
            }
        }

Elsewhere I'm calling

SpillageRiskDescriptions.Add("VENTILATE AREA");
SpillageRiskDescriptions.Add("DO NOT ALLOW SPILLAGE TO ENTER MAINS");

These seem to be adding elements to the private ArrayList _SpillageRiskDescriptions (through the property) whereas I would've expected this to cause a problem. Therefore am I correct in thinking that properties return a reference to the original variable and not passing it by value? Is this because ArrayList is a reference type? Will the same happen with an int (for example?)

Answer 1

Technically it's always by value, but you have to understand what is being passed. Since it's a reference type, you are passing a reference back (but by value).

Hope that makes sense. You always pass the result back by value, but if the type is a reference you are passing the reference back by value, which means you can change the object, but not which object it refers to.