How to update value in a List using LINQ

Question

I have a list which I want to update using LINQ.

class Student
{
    private string name;
    private int marks;
    public string Name { get; set;}
    public int Marks { get; set; }
    public Student(string name, int marks)
    {
        Name = name;
        Marks = marks;
    }
}

List<Student> myList = new List<Student>();
myList.Add(new Student("John", 10));
myList.Add(new Student("Tom", 20));

Now I want to update the list using LINQ such that only marks of John gets updated. I am using the following syntax:

myList.Where(w => w.Name == "Tom").Select(w=> { w.Marks = 35; return w});

But this doesnt update data in myList. Can someone tell me where am I going wrong.

Answer 1

LINQ is for querying, not for updating the data. Use LINQ to get the items that you want to modify, and then modify them in a foreach loop:

foreach ( var tom in myList.Where(w => w.Name == "Tom")) {
    tom.Marks = 35;
}

Demo.

Answer 2

Try:

myList .Where(w=> w.Name  == "dTomi").ToList().ForEach(i => i.Marks  = 35);

Answer 3

Select(), like any other LINQ method, doesn't change a source collection, instead, it return a new collection.

So you should either assign that new collection to myList:

myList = myList.Where(w => w.Name == "Tom").Select(w => { w.Marks = 35; return w}).ToList();

or assign Marks property in a separate foreach loop

myList.Where(w => w.Name == "Tom").ToList().ForEach(w => w.Marks = 35);

Answer 4

As others have pointed out, LINQ is for querying data, it is not for doing updates.

You should iterate your list, and modify your values like:

foreach (var student in myList)
{
    if (student.Name == "Tom")
    {
        student.Marks = 35;
    }
}

Or

foreach (var student in myList.Where(r => r.Name == "Tom"))
{
    student.Marks = 35;
}

Whatever you think better conveys the intent use that.

---

but here is an interesting thing:

If you have a statement like:

myList.Where(w => w.Name == "Tom").Select(w => w.Marks = 35).ToList();

Without assigning the result back to myList the above query will modify the value in the original list. Remember, it is a side effect and it is not the proper way to update. This is modification can be explained by reference parameter being passed to a method and getting modified there. But Importantly, this should always be avoided. It is a bad practice and could lead to really confusing code. Use LINQ for querying only.

Answer 5

Above can be achieved by just assign the value back to the collection

myList = myList
        .Where(w => w.Name == "Tom")
        .Select(w=> { w.Marks = 35; return w})
        .ToList();

Answer 6

how about old good for loop

for(int i = 0; i < myList.Count; i++)
if (myList[i].Name  == "Tom")
{
    myList[i].Marks = 35;
    break;
}

Answer 7

well @dasblinkenlight code should work but you can also try something like this

 var query = from x in list
             where x.Name = yourcondition
             select new { x };
foreach(var item in query)
   item.x.FieldToUpdate = SetValue;

Answer 8

This is kind of clunky, but works, and doesn't depend on being passed a reference. It creates a new list based on the old.

var myList=myList.Select(l=>new Student { 
   l.Name,
   Marks=l.Name=="Tom"?35:l.Marks}).ToList();

Or more silly:

var myList=myList.Where(l=>l.Name!="Tom").Union(
  myList.Where(l=>l.Name=="Tom").Select(l=>new Student { 
   l.Name,
   Marks=35})).ToList();

Answer 9

Objects are stored by reference in the list so you can recover object via linq and then edit, it will be reflect the changes on the list.

Example

    static void Main(string[] args)
        {
            List<Entity> testList = new List<Entity>()
            {
                new Entity() {Id = 1, Text = "Text"},
                new Entity() {Id = 2, Text = "Text2"}
            };

            Console.WriteLine($"First text value:{testList[1].Text}");

            Entity entityToEdit = testList.FirstOrDefault(e => e.Id == 2);
            if (entityToEdit != null)
                entityToEdit.Text = "Hello You!!";

            Console.WriteLine($"Edited text value:{testList[1].Text}");
            
            Console.ReadLine();
        }

 internal class Entity
    {
        public int Id { get; set; }
        public String Text { get; set; }
    }

Testing the app you will get the follow result:

First text value:Text2

Edited text value:Hello You!!

Answer 10

List can be updated with linq as sample code below, to avoid iteration :

        if (resultQuiz.ANSWER_Q != null) {
            questionsvm.Where(x => x.QUESTION_ID == resultQuiz.QUESTION_ID).FirstOrDefault().ANSWER_Q = resultQuiz.ANSWER_Q;

            questionsvm.Where(x => x.QUESTION_ID == resultQuiz.QUESTION_ID).FirstOrDefault().ISCORRECT =
                questionsvm.Where(x => x.QUESTION_ID == resultQuiz.QUESTION_ID).FirstOrDefault().ANSWER_TEXT.ToLower() == resultQuiz.ANSWER_Q.ToLower();             
        }