List passed by ref - help me explain this behaviour

Take a look at the following program:

class Test
{
    List<int> myList = new List<int>();

    public void TestMethod()
    {
        myList.Add(100);
        myList.Add(50);
        myList.Add(10);

        ChangeList(myList);

        foreach (int i in myList)
        {
            Console.WriteLine(i);
        }
    }

    private void ChangeList(List<int> myList)
    {
        myList.Sort();

        List<int> myList2 = new List<int>();
        myList2.Add(3);
        myList2.Add(4);

        myList = myList2;
    }
}

I assumed myList would have passed by ref, and the output would

3
4

The list is indeed "passed by ref", but only the sort function takes effect. The following statement myList = myList2; has no effect.

So the output is in fact:

10
50
100

Can you help me explain this behavior? If indeed myList is not passed-by-ref (as it appears from myList = myList2 not taking effect), how does myList.Sort() take effect?

I was assuming even that statement to not take effect and the output to be:

100
50
10

Solution 1:

Initially, it can be represented graphically as follow:

Then, the sort is applied myList.Sort();

Finally, when you did: myList' = myList2, you lost the one of the reference but not the original and the collection stayed sorted.

If you use by reference (ref) then myList' and myList will become the same (only one reference).

Note: I use myList' to represent the parameter that you use in ChangeList (because you gave the same name as the original)

Solution 2:

You are passing a reference to the list, but your aren't passing the list variable by reference - so when you call ChangeList the value of the variable (i.e. the reference - think "pointer") is copied - and changes to the value of the parameter inside ChangeList aren't seen by TestMethod.

try:

private void ChangeList(ref List<int> myList) {...}
...
ChangeList(ref myList);

This then passes a reference to the local-variable myRef (as declared in TestMethod); now, if you reassign the parameter inside ChangeList you are also reassigning the variable inside TestMethod.