List copied from same object, properties updated in both places

Question

I've done a lot of reading on copying/cloning objects in C# and I was trying to work from this example, but my situation is a little different.

I have a list of question/answer pairs being returned from the database. I want to use that to create two lists for parent and child objects. When I do that, any changes I make to the child are reflected in the parent.

I've tried several permutations of creating new lists, cloning, etc, but always end up with the same result.

List<QuestionVM> questionlist = productRepository.GetQuestions();
parent.Questions = AddQuestions(true, parent, questionlist);
child.Questions = AddQuestions(false, child, questionlist);

private List<QuestionVM> AddQuestions(bool parent, Line line, List<QuestionVM> questions)
{
    //shouldn't this create a new object, not just a reference?
    List<QuestionVM> qs = new List<QuestionVM>(questions);

    if (parent)
    {
        return qs.Where(w => w.ShowOnParent).ToList();
    }
    else
    {
        return qs.ToList(); 
    }
}

Any help is greatly appreciated.

It does create a new `List` object, but items in that list are references to the same items as in the initial list. — MarcinJuraszek, Apr 10 '15 at 17:48

score 2 · Accepted Answer · edited May 23 '17 at 11:50

2

It is not enough to just create copy of the list, because items in both old and new lists will still refer to the same objects.

You need to implement ICloneable interface(or just your own custom method) for your objects and call clone for each original object while creating copy list:

public class QuestionVM: ICloneable
{
     public Object Clone()
     {
          return new QuestionVM(this.prop1, this.prop2);
     }
}

...

private List<QuestionVM> AddQuestions(bool parent, Line line, List<QuestionVM> questions)
{
...
return qs
    .Where(w => w.ShowOnParent)
    .Select(obj => (VMQuestion)obj.Clone())
    .ToList();
...
}

P.S.: If VMQuestion contains fields of other complex types then they have to provide Clone method as well.

edited May 23 '17 at 11:50

Community

1
1

answered Apr 10 '15 at 17:49

Eugene Podskal

10,270
5
31
53

That was it! Thanks so much. Is there a difference between your Clone() method and using this: public object Clone() { return this.MemberwiseClone(); } – jenni-mm2 Apr 10 '15 at 18:12
1

@jlkmpls Yes, you can use `this.MemberwiseClone();` if VMQuestion constists of immutable members. It will be simpler than manual instantiation. But if members are of more complex mutable types then you will have to either augment `MemberwiseClone` [with some recursion](http://yangliucsharp.blogspot.ru/2013/10/deep-clone-vs-shallow-clone.html) or create new copy manually with specific constructors. – Eugene Podskal Apr 10 '15 at 18:38

score 0 · Answer 2 · answered Dec 01 '20 at 05:52

0

If anyone else is still interested in this. You can use automapper to make deepcopy of an object.

answered Dec 01 '20 at 05:52

Seble Nigussie

1

1

Would be great if you add some code using automapper (from the question, precisely), some basic instructions to get it working and some links for further documentation. – juagicre Dec 01 '20 at 07:43

List copied from same object, properties updated in both places

2 Answers2