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
    }
}

Solution 1

duplicate the list:

_myLocalList = new List<int>(_myList);

and perform the operations on the local list.

Solution 2

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()) .

Solution 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);
}

Solution 4

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();
        }
}

View more solutions

Comments

[Paligulus]

If the callers forgets to create a copy then they will get unexpected behaviour - I would prefer for the class\method to handle it

[Simon Stender Boisen]

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.

[newacct]

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