Value inside a list gets changed on modification C#

Question

Currently I have a class called Field:

public class Field
{
    public int Id { get; set; }
}

var listOfListOfFields = new List<List<Field>>();

var listOfFields = new List<Field>(){ new Field {id = 1}}; // in reality, data comes here.

for (int i = 0; i <= 2; i++)
{
var copyFirstList = listOfFields.ToList(); // to avoid reference thing, I still get same value
for(int j =0; j < copyFirstList.Count; j++)
{
 copyFirstList[j].Id = i;
}

listOfListOfFields.Add(copyFirstList);
}

The output in the listOfListOfFields contains 2 for all the lists in id. Please suggest.

This is nonsensical. tempIndex isn't defined. You're copying an empty list object. — John, Dec 03 '20 at 07:01
Actually not clear what do you want. However, just a guess - maybe you have a typo in `copyFirstList[tempIndex].Id = i;` and you want ` = j ` there? — Quercus, Dec 03 '20 at 07:12
With `listOfFields.ToList()` you create a copy of the list, but it still contains the same Fields (`copyFirstList[0]` is the same Field as `listOfFields[0]`) — Hans Keﬆing, Dec 03 '20 at 07:15
Copying a list doesn't create a copy of the values, unless they are structs. You're modifying the same item — Panagiotis Kanavos, Dec 03 '20 at 07:31
Does this answer your question? [How do you do a deep copy of an object in .NET?](https://stackoverflow.com/questions/129389/how-do-you-do-a-deep-copy-of-an-object-in-net) — Sinatr, Dec 03 '20 at 08:24

TheGeneral · Accepted Answer · 2020-12-03T07:35:11.527

4

Copying a list of references types gives you the same references to those reference types (nothing changed). For instance, if someone gave you a bucket of post-it-notes with phone numbers on them, and you tip them in to another bucket, you still have the same post-it-notes!

There are many ways to solve this problem. However, one way is to project using Select and recreating the objects

copyFirstList = listOfFields.Select(x => new Field() {Id = x.Id}).ToList():

edited Dec 03 '20 at 07:35

answered Dec 03 '20 at 07:29

TheGeneral

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thanks. this works. I also figured out ICloneable for the same. – Gauravsa Dec 03 '20 at 07:34

Value inside a list gets changed on modification C#

1 Answers1