Pass List to method without modifying original list

Question

Is this the only way of passing a List to a method and editing that List, without modifying the original List?

class CopyTest1
{
    List<int> _myList = new List<int>();
    public CopyTest1(List<int> l)
    {
        foreach (int num in l)
        {
            _myList.Add(num);
        }
        _myList.RemoveAt(0); // no effect on original List
    }
}

Is this `a` way of passing a List to a method and editing that List, without modifying the original List? — Bastardo, Oct 10 '11 at 12:39

score 18 · Accepted Answer · answered Oct 10 '11 at 12:40

18

duplicate the list:

_myLocalList = new List<int>(_myList);

and perform the operations on the local list.

answered Oct 10 '11 at 12:40

Andreas

6,447
2
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score 6 · Answer 2 · answered Oct 10 '11 at 12:43

Use AsReadOnly for this:

class CopyTest1
{
    List<int> _myList = new List<int>();
    public CopyTest1(IList<int> l)
    {
        foreach (int num in l)
        {
            _myList.Add(num);
        }
        _myList.RemoveAt(0); // no effect on original List
    }
}

And call it via CopyTest1(yourList.AsReadOnly()) .

score 3 · Answer 3 · answered Oct 10 '11 at 12:42

3

There is another way. You can use the copy constructor of List<T>:

List<int> _myList;
public CopyTest1(List<int> l)
{
    _myList = new List<int>(l);
}

answered Oct 10 '11 at 12:42

Ilian

5,113
1
32
41

score 1 · Answer 4 · answered Oct 10 '11 at 12:41

Clone objects in the list to other list and work on this copy

static class Extensions
{
        public static IList<T> Clone<T>(this IList<T> listToClone) where T: ICloneable
        {
                return listToClone.Select(item => (T)item.Clone()).ToList();
        }
}

score 0 · Answer 5 · answered Oct 10 '11 at 12:43

When you pass a list to a method you pass a pointer to said list, that's why your changing the 'original' list when you modify it inside your method. If you instead want to modify a copy of the list you just need to make one. In the code that calls CopyTest1 you can create a new list based on your original list:

public void CallsCopyTest1()
{
    var originalList = new List<int>();
    var newList = new List<int>(originalList);
    var copyTest = new CopyTest1(newList); //Modifies newList not originalList
}
class CopyTest1
{
    List<int> _myList = new List<int>();
    public CopyTest1(List<int> l)
    {
        foreach (int num in l)
        {
            _myList.Add(num);
        }
        _myList.RemoveAt(0); // no effect on original List
    }
}

If the callers forgets to create a copy then they will get unexpected behaviour - I would prefer for the class\method to handle it — Paligulus, Oct 10 '11 at 12:47
That depends what the purpose of the class is, is it do modify a collection or to create a copy of a collection and modify it? The example is a bit contrived and I don't think it's obvious exactly what you wanted. — Simon Stender Boisen, Oct 10 '11 at 12:52

score -1 · Answer 6 · answered Oct 10 '11 at 12:45

-1

You can pass an object by reference doing the following:

public static void ReferenceMethod(ref List<T> myParam) {
    ...
}

EDIT: The question has now been clarified, the OP was after a way not to alter the original list.

answered Oct 10 '11 at 12:45

dougajmcdonald

19,231
12
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and in any case it is not necessary to pass it as `ref` to alter the original list -- `ref` allows you to change the reference in the caller's scope to point to another object, something that is highly unlikely to be useful – newacct Oct 11 '11 at 04:29

Pass List to method without modifying original list

6 Answers6

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