List passed by ref - help me explain this behaviour

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.

Here is an easy way to understand it

• Your List is an object created on heap. The variable myList is a reference to that object.

• In C# you never pass objects, you pass their references by value.

• When you access the list object via the passed reference in ChangeList (while sorting, for example) the original list is changed.

• The assignment on the ChangeList method is made to the value of the reference, hence no changes are done to the original list (still on the heap but not referenced on the method variable anymore).

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)