c# - List passed by ref - help me explain this behaviour

Question

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

Answer 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)