Keeping the original list unchanged

Question

In my following code, myOriginalList is still getting modified. How do I keep it unchanged?

Thanks for your help.

List<Product> myOriginalList = GetList(); 
List<Product> customList= new List<Product>();

                        foreach (var productType in myProductTypeList )
                        {
                            var tempList = myOriginalList.ToList(); 
                            var result = GetModifiedCustomList(tempList, productType );
                            customList.AddRange(result);                                                                                    
                        }
.....
.....
 private List<Product> GetModifiedCustomList(List<Product> inplist, string productType)
        {
            List<Product> tmpList = new List<Product>(inplist); 

            if (tmpList?.Count > 0)
            {
                tmpList.ForEach(r => { if (r.ProductType == "NONE") { r.ProductType= productType; } }); 
            }
            return tmpList; 
        }

Possible duplicate of [Pass List to method without modifying original list](https://stackoverflow.com/questions/7712842/pass-list-to-method-without-modifying-original-list) — Jerodev, Apr 25 '18 at 09:36
I don't think that `myOriginalList.ToList()` is creating a new list; it's passing the same reference. Try changing it to `myOriginalList.Select(m => m).ToList()` or `new List(myOriginalList)`. It will create a new copy of myOriginalList. — user1672994, Apr 25 '18 at 09:36
As I see the code, the old list is not modified, but the values in the list are modified, since you change the instance value from new list. — Julo, Apr 25 '18 at 09:37
@jerodev no..it isn't. That is for value types where new List(_list) creates a new copy. For reference types, reference gets copied — Skyuppercut, Apr 25 '18 at 09:38
Yes, it's not your list that is being modified, it's the objects in the list. — DavidG, Apr 25 '18 at 09:43

score 1 · Answer 1 · answered Apr 25 '18 at 09:43

You'll need to create a deep copy of the list. One possible way is to clone each of the Product object.

Add a static Clone method in the Product class which does similar function.

 public static Product Clone(Product product)
        {
            return new Product
            {
                Underlying = product.Underlying
            };
        }

Now, Iterate through the list, clone and add each element to the new list.

var newList = new List<Product>();
oldList.ForEach(x => newList.Add(x.Clone()));

score 0 · Answer 2 · answered Apr 25 '18 at 09:48

0

try this var tempList = new List<Product>(myOriginalList.ToArray()); as it will create new copy

answered Apr 25 '18 at 09:48

Ravi Mungalpara

14
5

Thanks @Ravi but no luck...ProductType property in my myOriginalList still gets modified. – Jimmy Apr 25 '18 at 09:52
Still copies the references to the original objects. – Ahmad Ibrahim Apr 25 '18 at 09:53
1

One lengthy way is: var tempList = new List(myOriginalList.Count); foreach (Product item in myOriginalList) tempList.Add((Product)item.Clone()); return tempList; – Ravi Mungalpara Apr 25 '18 at 10:05

Keeping the original list unchanged

2 Answers2