Passing objects by reference or not in C#

Question

Suppose I have a class like this:

public class ThingManager {
    List<SomeClass> ItemList;

    public void AddToList (SomeClass Item)
    {
        ItemList.Add(Item);
    }

    public void ProcessListItems()
    {
        // go through list one item at a time, get item from list,
        // modify item according to class' purpose
    }
}

Assume "SomeClass" is a fairly large class containing methods and members that are quite complex (List<>s and arrays, for example) and that there may be a large quantity of them, so not copying vast amounts of data around the program is important.

Should the "AddToList" method have "ref" in it or not? And why?

It's like trying to learn pointers in C all over again ;-) (which is probably why I am getting confused, I'm trying to relate these to pointers. In C it'd be "SomeClass *Item" and a list of "SomeClass *" variables)

Answer 1

If you ever need to use the original value of the parameter user ref. If not, use out. For reference:

http://www.yoda.arachsys.com/csharp/parameters.html

http://msdn.microsoft.com/en-us/library/0f66670z(VS.71).aspx

Answer 2

This is an excellent overview of parameter passing in C#.

Answer 3

Since SomeClass is a reference type, you do not need to use the "ref" keyword. If it were a value type, "ref" might be useful.

Answer 4

Since SomeClass is a class, then it is automatically passed by reference to the AddToList method (or more accurately, its reference is passed by value) so the object is not copied. You only need to use the ref keyword if you want to re-assign the object the reference points to in the AddToList method e.g. Item = new SomeClass();.

Answer 5

Think of out as a way of making a parameter work as a return value.

So these are very similar:

void Foo(out int result)
{
    result = 5;
}

int Foo()
{
    return 5;
}

And then think of ref as a way of allowing a parameter to be both an input and an output.

So in your example, if you declared your method:

public void AddToList(ref SomeClass Item)

Then the caller would have to write something like:

SomeClass i = null;
obj.AddToList(ref i);

This would be illegal, for example:

obj.AddToList(ref new SomeClass());

They would be forced to pass a variable name, rather than an expression, so that the AddToList method can store a value in the variable. By adding the ref prefix you are allowing your method to make the passed variable point to a different object.