Best way to update an element in a generic List

Question

Suppose we have a class called Dog with two strings "Name" and "Id". Now suppose we have a list with 4 dogs in it. If you wanted to change the name of the Dog with the "Id" of "2" what would be the best way to do it?

Dog d1 = new Dog("Fluffy", "1");
Dog d2 = new Dog("Rex", "2");
Dog d3 = new Dog("Luna", "3");
Dog d4 = new Dog("Willie", "4");

List<Dog> AllDogs = new List<Dog>()
AllDogs.Add(d1);
AllDogs.Add(d2);
AllDogs.Add(d3);
AllDogs.Add(d4);

@SemiDemented Best meaning? Best in what? Performance? Memory? Maintenance? Concise? or what? Best means little, you should be specific. — Sriram Sakthivel, Sep 03 '14 at 11:23

Mike Perrenoud · Accepted Answer · 2018-05-16T09:58:49.483

98

AllDogs.First(d => d.Id == "2").Name = "some value";

However, a safer version of that might be this:

var dog = AllDogs.FirstOrDefault(d => d.Id == "2");
if (dog != null) { dog.Name = "some value"; }

edited May 16 '18 at 09:58

answered Oct 09 '13 at 19:35

Mike Perrenoud

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Note that you can also just do `AllDogs.First(d => d.Id == "2").Name =`... – Reed Copsey Oct 09 '13 at 19:36
@ReedCopsey, thank you very much for that. A more concise way of getting at it! – Mike Perrenoud Oct 09 '13 at 19:36
4

This would work if you have one property to update. What if there are many properties and you want to replace the entire object in the list with a new one? – user20358 Jan 06 '16 at 14:08
1

Why not use `var dog = AllDogs.FirstOrDefault(d => d.Id == "2");` instead of `var dog = AllDogs.Where(d => d.Id == "2").FirstOrDefault();` ? – Vitaly Filatenko May 15 '18 at 11:32
@KotMatroskin, yep that's a better optimization. Edited. – Mike Perrenoud May 16 '18 at 09:59

gleng · Answer 2 · 2016-04-04T19:23:40.130

7

You could do:

var matchingDog = AllDogs.FirstOrDefault(dog => dog.Id == "2"));

This will return the matching dog, else it will return null.

You can then set the property like follows:

if (matchingDog != null)
    matchingDog.Name = "New Dog Name";

edited Apr 04 '16 at 19:23

answered Oct 09 '13 at 19:36

gleng

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`FirstOrDefault` isn't really useful here. If it returns "default", it'll raise a `NullReferenceException` when you try to set `Name`... – Reed Copsey Oct 09 '13 at 19:36
2

that means it really IS useful here because if you just use .first, you'll get an exception doing the select itself. – John Lord Oct 22 '20 at 18:58

score 0 · Answer 3 · answered Oct 09 '13 at 20:14

If the list is sorted (as happens to be in the example) a binary search on index certainly works.

    public static Dog Find(List<Dog> AllDogs, string Id)
    {
        int p = 0;
        int n = AllDogs.Count;
        while (true)
        {
            int m = (n + p) / 2;
            Dog d = AllDogs[m];
            int r = string.Compare(Id, d.Id);
            if (r == 0)
                return d;
            if (m == p)
                return null;
            if (r < 0)
                n = m;
            if (r > 0)
                p = m;
        }
    }

Not sure what the LINQ version of this would be.

Why reinvent that. [List.BinarySearch](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=2ahUKEwjMiZf1zMblAhXjQ98KHWYUCMwQFjAAegQIBRAG&url=https%3A%2F%2Flearn.microsoft.com%2Fen-us%2Fdotnet%2Fapi%2Fsystem.collections.generic.list-1.binarysearch&usg=AOvVaw2CwInMFi1iu3vXlCBu52yj) works great. — Suncat2000, Oct 31 '19 at 13:28

Best way to update an element in a generic List

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