Replace an object in a list of objects

Question

In C#, if I have a List<T>, and I have an object of type T, how can I replace a specific item in the List<T> with the object of type T?

Here is what I have tried:

List<CustomListItem> customListItems = new List<CustomListItem>();
CustomListItem customListItem1 = new CustomListItem() { name = "Item 1", date = DateTime.MinValue};
CustomListItem customListItem2 = new CustomListItem() { name = "Item 2", date = DateTime.MinValue };
CustomListItem customListItem3 = new CustomListItem() { name = "Item 3", date = DateTime.MinValue };

customListItems.Add(customListItem1);
customListItems.Add(customListItem2);
customListItems.Add(customListItem3);

CustomListItem newCustomListItem = new CustomListItem() { name = "Item 4", date = DateTime.Now };

customListItem2 = customListItems.Where(i=> i.name == "Item 2").First();
customListItem2 = newCustomListItem;

In the above code, I want to replace the customListItem2 with the newCustomListItem.

Do I have to remove the item in the list, and then insert the new item? Can I not do a simple assignment of customListItem2 = newCustomListItem?

What is the most efficient way of replacing an item in a list with another item?

Brendabrendan

Have you checked #17189466? – Essieessinger

Additional to the solutions that have already been posted you should also implement an Equals/GetHashCode for your custom type, otherwise your Where/Index-operation may fail to find the right element. – Zareba

Answer 1

You have to replace the item, not the value of customListItem2. Just replace following:

customListItem2 = customListItems.Where(i=> i.name == "Item 2").First();
customListItem2 = newCustomListItem;

With this:

customListItem2 = customListItems.Where(i=> i.name == "Item 2").First();
var index = customListItems.IndexOf(customListItem2);

if(index != -1)
    customListItems[index] = newCustomListItem;

Edit:

As Roman R. stated in a comment, you can replace the .Where(predicate).First() by a simple First(predicate):

customListItem2 = customListItems.First(i=> i.name == "Item 2");

Stringed

You can replace the .Where with .First in the first place and delete it from the end. It will be the same result but with cleaner and faster code. Do it like the following: customListItem2 = customListItems.First(i=> i.name == "Item 2"); – Cristobalcristobalite

Using this code, is it possible that the index will be -1? After all, if it wasn't on the list, First() would throw an error first :) – Fabianfabianism

then use FirstOrDefault() which will return null if not found, then -1 for the IndexOf(null) – Howler

@RomanR. - It will make it only cleaner; it's already proven that First(expresion) is slower than Where(expresion).First(). Useful link: #8664397 – Dilatory

@Dilatory seems to be outdated. Have a look at this dotnetfiddle.net/OrUUSG Tried it myself and verified the results. – Cristobalcristobalite

EDIT: Depends on the parameters your passing. Change n to 90 and First without Where is slower... – Cristobalcristobalite

@Dilatory interesting observation - visual studio will try to underline it and suggest that you make it .First instead of .Where().First though despite the slowness – Kuo

Answer 2

var customListItems = new List<CustomListItem>();
var customListItem1 = new CustomListItem() { name = "Item 1", date = DateTime.MinValue };
var customListItem2 = new CustomListItem() { name = "Item 2", date = DateTime.MinValue };
var customListItem3 = new CustomListItem() { name = "Item 3", date = DateTime.MinValue };

customListItems.Add(customListItem1);
customListItems.Add(customListItem2);
customListItems.Add(customListItem3);

var newCustomListItem = new CustomListItem() { name = "Item 4", date = DateTime.Now };

customListItems[customListItems.FindIndex(x => x.name == "Item 2")] = newCustomListItem;

or

public static class ListExtensions
{
    public static void Replace<T>(this List<T> list, Predicate<T> oldItemSelector , T newItem)
    {
        //check for different situations here and throw exception
        //if list contains multiple items that match the predicate
        //or check for nullability of list and etc ...
        var oldItemIndex = list.FindIndex(oldItemSelector);
        list[oldItemIndex] = newItem;
    }
}

and then

customListItems.Replace(x => x.name == "Item 2", newCustomListItem);

Blancheblanchette

Does the extension approach use reflection? It is certainly more convenient with this code, and I went this route. Just wondering about any performance hit. – Stallard

No, it does not use reflection. @Stallard – Blancheblanchette

With this solution the list is only looped once. With the one in the accepted answer twice. But often or usually the not-found-case should be handled. If the item is not found FindIndex returns -1. – Lingam

Answer 3

if sequence of list not matter to you then you can try this

CustomListItem newCustomListItem = new CustomListItem() { name = "Item 4", date = DateTime.Now };

customListItem2 = customListItems.Where(i=> i.name == "Item 2").First();  

customListItems.Remove(customListItem2);
customListItems.Add(newCustomListItem );

Smolt

Answer 4

You can use a select to build a new iEnumerable with replace item :

customListItems.Select(x => x.name == "Item 2" ? newItem : x).ToList();

i did not test for performance but I found this solution quite concise and explicit.

Analemma

Answer 5

An alternative would be to use the DynamicData package that has lot of others facilities on lists:

Quick Answer

make sure the package is installed:

Install-Package DynamicData -Version 7.4.3

then

using DynamicData;

DynamicData.ListEx.Replace<CustomListItem>(customListItems, customListItem2, newCustomListItem);

The following is the complete working solution.

Working Example

my custom item class

public class CustomListItem
{
    public string name; 
    public DateTime date;

    public override string ToString()
    {
        return name + ", " + date.ToString();
    }
}

In the main try this, make sure to comment out the assignment before the replacement.

static void Main(string[] args)
{
    List<CustomListItem> customListItems = new List<CustomListItem>();
    CustomListItem customListItem1 = new CustomListItem() { name = "Item 1", date = DateTime.MinValue };
    CustomListItem customListItem2 = new CustomListItem() { name = "Item 2", date = DateTime.MinValue };
    CustomListItem customListItem3 = new CustomListItem() { name = "Item 3", date = DateTime.MinValue };

    customListItems.Add(customListItem1);
    customListItems.Add(customListItem2);
    customListItems.Add(customListItem3);

    CustomListItem newCustomListItem = new CustomListItem() { name = "Item 4", date = DateTime.Now };

    customListItem2 = customListItems.Where(i => i.name == "Item 2").First();
    //customListItem2 = newCustomListItem;


    DynamicData.ListEx.Replace<CustomListItem>(customListItems, customListItem2, newCustomListItem);

    foreach(var customListItem in customListItems)
    {
        Debug.Print("\n" + customListItem.ToString());
    }
}

references: https://github.com/reactivemarbles/DynamicData.
https://dynamic-data.org

Putrefy

Answer 6

I normally have some basic interfaces that define a key field so that I can use generics more easily. So classes that inherit IKey will have a string Key field that is a unique identifier to the record.

So building ontop of Hamid's example but with a 'add or replace' concept:

    public static void AddOrReplace<T>(this List<T> items, T newItem) where T : class, IKey
    {
        items.AddOrReplace(x => x.Key == newItem.Key, newItem);
    }

    public static void AddOrReplace<T>(this List<T> items, Predicate<T> oldItemSelector, T newItem)
    {            
        var index = items.FindIndex(oldItemSelector);
        if (index < 0)
        {
            items.Add(newItem);
        }
        else
        {
            items[index] = newItem;
        }            
    }

Then to implement these extensions I can use a similar syntax to the list.Add():

list.AddOrReplace(newItem);

Moorfowl

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