Deep copy of a list with class elements

Question

EDIT: To whoever marked the question as duplicate. That question is for how to create a deep copy. My question was how to make sure a the copy constructor is called when copying a list of class elements.

I'm trying to make a deep copy of a List that contain custom class elements. If I have a List of strings I can just use

List<string> secondList = new List<string>(firstList);

and then freely modify the elements in the second list without effeting the ones in the firwst list. But when I try to do the same with a custom class type both lists get changed. To try and solve it I made a small test program that just has this class.

class TestClass
{
    public string name;

    public TestClass(string n)
    {
        name = n;
    }

    public TestClass(TestClass original)
    {
        name = original.name;
    }
}

And all my program does is this

TestClass t = new TestClass("Name1");
List<TestClass> list1 = new List<TestClass>();
list1.Add(t);

List<TestClass> list2 = new List<TestClass>(list1);
list2[0].name = "Name2";

That last line of code changes the name of the first element in both lists, which I do no want.

Possible duplicate of [Deep cloning objects](http://stackoverflow.com/questions/78536/deep-cloning-objects) — Ben, Dec 07 '15 at 14:27

Matthew Watson · Accepted Answer · 2015-12-07T14:34:35.983

The issue here is that your objects are reference types, and the lists hold references to those objects.

This means that even though your second list has a COPY of the references from the first list, the references are still pointing to the original objects.

In order to solve this, you must clone not the references in the lists but instead the actual objects that you have stored in the lists.

You have already defined a copy constructor for your class, so you can use that to make a deep copy of the list as follows:

var list2 = list1.Select(item => new TestClass(item)).ToList();

score 1 · Answer 2 · answered Dec 07 '15 at 14:42

You create a reference with this line of Code:

List<TestClass> list2 = new List<TestClass>(list1);

But you won't like to use Call-by-Reference. You Need Call-by-Value in this Approach.

so the working code in lambda-expression is the following one:

        TestClass t = new TestClass("Name1");
        List<TestClass> list1 = new List<TestClass>();
        list1.Add(t);

        List<TestClass> list2 = new List<TestClass>();
        list2 = list1.Select(item => new TestClass(item)).ToList();
        list2[0].name = "Name2";

Have fun with it...

Deep copy of a list with class elements

2 Answers2