How to copy a List<T> without cloning

Question

Perhaps I am missing something trivial. I have a couple of List<T>s and I need one big list from them which is a union of all the other lists. But I do want their references in that big list and not just the values/copies (unlike many questions I typically find on SO).

For example I have this,

List<string> list1 = new List<string> { "a", "b", "c" };
List<string> list2 = new List<string> { "1", "2", "3" };

var unionList = GetThatList(list1, list2);

Suppose I get the list I want in unionList, then this should happen:

unionList.Remove("a"); => list1.Remove("a");
unionList.Remove("1"); => list2.Remove("1");

//in other words
//
//unionList.Count = 4;
//list1.Count = 2;
//list2.Count = 2;

To make it clear, this typically happens with

unionList = list1; //got the reference copy.

But how do I go about with the second list, list2 to add to unionList?

I tried Add and AddRange but they obviously clone and not copy.

unionList = list1;
unionList.AddRange(list2); //-- error, clones, not copies here.

and

foreach (var item in list2)
{
    unionList.Add(item); //-- error, clones, not copies here.
}

Update: I think I am asking something that makes no sense, and something that inherently is not possible in the language..

Answer 1

I don't think any such class exists. You could implement it yourself. Here's a start:

class CombinedLists<T> : IEnumerable<T> // Add more interfaces here.
                                        // Maybe IList<T>, but how should it work?
{
    private List<List<T>> lists = new List<List<T>>();

    public void AddList(List<T> list)
    {
        lists.Add(list);
    }

    public IEnumerator<T> GetEnumerator()
    {
        return lists.SelectMany(x => x).GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public bool Remove(T t)
    {
        foreach (List<T> list in lists)
        {
            if (list.Remove(t)) { return true; }
        }
        return false;
    }

    // Implement the other methods.
}

Here's some code you can use to test it:

List<string> list1 = new List<string> { "a", "b", "c" };
List<string> list2 = new List<string> { "1", "2", "3" };
CombinedLists<string> c = new CombinedLists<string>();
c.AddList(list1);
c.AddList(list2);

c.Remove("a");
c.Remove("1");

foreach (var x in c) { Console.WriteLine(x); }
Console.WriteLine(list1.Count);
Console.WriteLine(list2.Count);

Removing items is fairly simple. But you might get problems if you try to insert items into your combined list. It is not always well-defined which list should receive the inserted items. For example, if you have a combined list that contains two empty lists and you insert an item at index 0, should the item be added to the first or the second empty list?