C# facing problem to add data into List<T> from LINQ select function

Question

C# facing problem to add data into List<T> from LINQ select function

T.Zacks 3,996

please have a look this code

            List<string> lst = new List<string>();
            IEnumerable<int> squares = Enumerable.Range(0, 3).Select(x => new
            {
                lst.Add(x.ToString())
            });

within select function can't we add anything to List<T> ?

where i made the mistake in code? please tell me how to achieve what i am trying to do. thanks

2 answers

Your answer

Answer 1

P a u l 10,761

Try this instead:

foreach (var x in Enumerable.Range(0, 3)) {
    lst.Add(x.ToString());
}

The .Select() method is for projecting/mapping from one set of values to another, so the function that you pass to it need to return a value. In your case you're returning an anonymous object (i.e. new { }) but you're calling lst.Add() inside the body of the object which is a syntax error.

You could also do it in one line like this:

List<string> lst = Enumerable.Range(0, 3).Select(x => x.ToString()).ToList();

T.Zacks 3,996 Reputation points

2022-02-04T15:49:30.427+00:00

from select() function can't we add value into List<T> ? i am curious to know how could i write normal statement inside select() function?

is it no way possible ?
P a u l 10,761 Reputation points

2022-02-04T16:00:42.917+00:00
You can, but that goes against what .Select() is designed for. It's for taking a collection and transforming it directly into a new collection by defining a transformation/projection function which will take the old item at every iteration & transform it into a new value.

The result of a .Select() call is an entirely new collection, so you don't need to create a list and add to it inside the body of your function, you can just use the result of the .Select() and that's your new collection like I added above in the one-line example.

It's possible that you can have an expression bodied function that you pass to .Select(), but it should only interact with the input of that function, not with objects defined outside of that scope. For example you have a source collection called numbers, and a new collection that's being produced by transforming the original values into new values (transformedNumbers):

int[] numbers = new[] { 1, 2, 3, 4, 5 }; IEnumerable<int> transformedNumbers = numbers.Select(n => { if (n == 3) { n *= 2; } return n; }); foreach (var number in transformedNumbers) { Console.WriteLine(number); }

Answer 2

Karen Payne MVP 35,586 Volunteer Moderator

Seems this would provide what you are after

List<string> stringList = new();
List<int> intList = new();

void AddToList(int value)
{
    stringList.Add(value.ToString());
    intList.Add(value);
}

Enumerable.Range(0, 3).ToList().ForEach(AddToList);

Or

var intArray = Enumerable.Range(0, 3).Select(x => x).ToArray();
string[] strArray = Array.ConvertAll(intArray, value => value.ToString());

Neither are one liners but that should not matter

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C# facing problem to add data into List<T> from LINQ select function

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