Why are changes made in foreach for grouping Linq ignored unless I add ToList ()?

Question

I have the following method.

public IEnumerable<Item> ChangeValueIEnumerable()
    {
        var items = new List<Item>(){
            new Item("Item1", 1),
            new Item("Item2", 1),
            new Item("Item3", 2),
            new Item("Item4", 2),
            new Item("Item5", 3)
        };

        var groupedItems = items.GroupBy(i => i.Value)
            .Select(x => new Item(x.First().Name, x.Key));

        foreach (var item in groupedItems)
        {
            item.CalculatedValue = item.Name + item.Value;
        }

        return groupedItems;
    }

In the collection, the groupedItemsvalue CalculatedValueis null. However, if I add ToList()to the sentence Selectafter GroupBy, the value CalculatedValuematters. eg:

 var groupedItems = items.GroupBy(i => i.Value)
            .Select(x => new Item(x.First().Name, x.Key)).ToList();

So the question is. Why is this? I want to know the reason for this, the solution for me is to addToList()

Update. Class deviation Itemfollowing

 public class Item
{
    public string Name { get; set; }
    public int Value { get; set; }

    public string CalculatedValue { get; set; }

    public Item(string name, int value)
    {
        this.Name = name;
        this.Value = value;
    }
}

Answer 1

var groupedItems = items.GroupBy(i => i.Value)
    .Select(x => new Item(x.First().Name, x.Key));

groupedItems . IEnumerable<T>, Select, - , items , x => new Item(x.First().Name, x.Key).

, groupedItems, .

var groupedItems = items.GroupBy(i => i.Value)
    .Select(x => 
    {
        Console.WriteLine("Creating new item");
        return new Item(x.First().Name, x.Key));
    }

foreach(var item in groupedItems);
foreach(var item in groupedItems);

, , " " items.

CalculatedValue . foreach , .

ToList, "" .

ToList , CalculatedValue. .

Func<Item, Item> withCalculatedValue =
    item => {
        item.CalculatedValue = item.Name + item.Value;
        return item;
    };

return items.GroupBy(i => i.Value)
        .Select(x => new Item(x.First().Name, x.Key))
        .Select(withCalculatedValue);

return items.GroupBy(i => i.Value)
        .Select(x => new Item(x.First().Name, x.Key) { CalculatedValue = x.First().Name + x.Key });

, , google "", .

Answer 2

dcastro, , (), for, groupedItems, , ChangeValueIEnumerable IEnumerable, groupedItems .

, ToList(), Select:

var groupedItems = items.GroupBy(i => i.Value)
    .Select(x => new Item(x.First().Name, x.Key)
    {
        CalculatedValue = x.First().Name + x.Key
    });

return groupedItems;