Question

I wonder if it’s possible to add an object from one list to another list like this:

objectToRemoveList.Add(listOfObjects.ElementAt(i));

Then, I want to remove all object in the list like this:

objectToRemoveList.Clear();

I wonder, because it seems like the objects are still there!?

Answer 1

That won’t work. Let me explain it in a graphical way. Your objects exist somewhere in memory:

  a     b     c     d

Your list listOfObjects only contains a reference to each of these objects:

listOfObjects:  |     |     |     |     
                |     |     |     |     
                v     v     v     v     
                a     b     c     d

If you add some of these items (let’s say b and d) to objectToRemoveList, you are also only adding a reference:

listOfObjects:  |     |     |     |     
                |     |     |     |     
                v     v     v     v     
                a     b     c     d
                      ^           ^
                      |           |
objToRemoveLst:       |           |

By executing objectToRemoveList.Clear, you are just removing all references there. Note how listOfObjects is completely unafffected:

listOfObjects:  |     |     |     |     
                |     |     |     |     
                v     v     v     v     
                a     b     c     d


objToRemoveLst:       

---

To achieve what you want to do, you can use the following loop:

foreach (object o in objectToRemoveList) {
    listOfObjects.Remove(o);
}

That way, the references are actually removed from listOfObjects, which is what you want:

listOfObjects:  |           |           
                |           |           
                v           v           
                a     b     c     d
                      ^           ^
                      |           |
objToRemoveLst:       |           |