LINQ retains unwanted data binding

Question

Let's make a variable named i_Changes of type List<List<int>> which has the following values:

[1] = 1,2
[2] = 3,4,6
[3] = 4,8,15

This Variable is stored in a class myClass which looks like this:

Public Class myClass
{
    public DateTime selectedDate {get; set}
    public List<List<int>> i_Changes {get;set;}
}

Now we have another variable named i_Holder of type List<List<List<int>>> which is supposed to hold values of i_Changes (that means after every iteration I want to store exact numbers from i_Changes so that later (after process) I can show them all to user in a Grid.

What I do after every iteration, to store new variable is the following:

i_Holder.Add(myClass.Select(n=>n.i_Changes).ToList();

I found out that when I add items to my list like this, they keep binding with the original i_Changes, which means that after every iteration (when values under i_Changes change), all values under i_Holder change accordingly -> that means I have list of n items with exact same values such as current i_Changes.

How can I solve this problem?

Also, if you believe that List<List<List<int>>> is not a good idea, please feel free to correct me, as it was the only thing what I could think of.

Thank you

EDIT: I found out that when I create new List<List<int>> and manually copy each values to that list, then binding is lost; however I believe it is not the best thing to do (create new List after every iteration). Is there any better way to do this?

code that does the operation has more than 600 lines. At the end of iteration I do exactly this (add new values to the list), so there is really no other relevant code — Robert J., Sep 12 '14 at 08:52
for `i_Holder`,I suggest you use `List`. and `i_Holder=i_Holder.Contact(myClass.SelectMany(n=>n.i_Changes).ToList());` — Tim.Tang, Sep 12 '14 at 08:52
what does .Contact stand for? I cannot find it among LINQ commands; also Select many would simply put results in 1D array if I am not mistaken; but I need to keep the structure of my array — Robert J., Sep 12 '14 at 08:55
Your sample code is really strange : `i_Holder.Add(myClass.Select(n=>n.i_Changes).ToList();` first it doesn't compile, secondly if `myClass` is an instance of `myClass`, you can't do a `Select` on it : it's not an `IEnumerable` — Raphaël Althaus, Sep 12 '14 at 09:12

Baldrick · Accepted Answer · 2014-09-12T11:51:32.620

2

You are holding a list of references to the same object, so when the object changes, you will see that change in every item in your list.

You need to create a new list containing the desired values each time, like this:

i_Holder.Add(myClass
    .Select(n => n.i_Changes.Select(j => new List<int>(j)).ToList()).ToList());

edited Sep 12 '14 at 11:51

answered Sep 12 '14 at 09:09

Baldrick

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That did the trick, thank you! However do you think that this is a good practice, creating new List every time? Shouldn't I create something different for this purpose? Or apply different method? – Robert J. Sep 12 '14 at 09:16
1

Well.. you need to make a copy of all the `int` values however you do it, so using a new `List` is probably no worse than any other approach! – Baldrick Sep 12 '14 at 09:18

score 1 · Answer 2 · edited May 23 '17 at 11:57

1

In C# lists are passed by reference so i think this is the reason you getting this issue. in order to avoid this you have to do deep copy of your lists, prior to adding them.

You can read about deep clone of the lists here

edited May 23 '17 at 11:57

Community

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answered Sep 12 '14 at 09:13

Alex Art.

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LINQ retains unwanted data binding

2 Answers2