Lambda expressions - set the value of one property in a collection of objects based on the value of another property in the collection

Question

I'm new to lambda expressions and looking to leverage the syntax to set the value of one property in a collection based on another value in a collection

Typically I would do a loop:

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

void Run()
{
    Item item1 = new Item { Name = "name1" };
    Item item2 = new Item { Name = "name2" };
    Item item3 = new Item { Name = "name3" };

    Collection<Item> items = new Collection<Item>() { item1, item2, item3 };

    // This is what I want to simplify.
    for (int i = 0; i < items.Count; i++)
    {
        if (items[i].Name == "name2")
        {
            // Set the value.
            items[i].Value = "value2";
        }
    }
}

Fourposter

Answer 1

LINQ is generally more useful for selecting data than for modifying data. However, you could write something like this:

foreach(var item in items.Where(it => it.Name == "name2")) 
  item.Value = "value2";

This first selects items that need to be modified and then modifies all of them using a standard imperative loop. You can replace the foreach loop with ForAll method that's available for lists, but I don't think this gives you any advantage:

items.Where(it => it.Name == "name2").ToList()
     .ForEach(it => it.Value = "value2");

Note that you need to add ToList in the middle, because ForEach is a .NET 2.0 feature that's available only for List<T> type - not for all IEnumerable<T> types (as other LINQ methods). If you like this approach, you can implement ForEach for IEnuerable<T>:

public static void ForEach<T>(this IEnumerable<T> en, Action<T> f) {
  foreach(var a in en) f(a);
}

// Then you can omit the `ToList` conversion in the middle 
items.Where(it => it.Name == "name2")
     .ForEach(it => it.Value = "value2");

Anyway, I'd prefer foreach loop, because that also makes it clear that you're doing some mutation - and it is useful to see this fact easily in the code.

Deuteragonist

beautiful. No way around the loop then? – Fourposter

@Michael Rut, the loop reads better than the ForEach method, but that's a matter of opinion. I agree with what Tomas said in his answer about the clarity of the action. However, he did show a way for you to abstract away the loop (although clearly a loop will still happen, rather you explicitly code it or not). – Infernal

If i want to update db after setting the value, which one would be appropriate for lesser hit on db ? – Blondellblondelle

The performance will be the same - they will all generate equal number of UPDATE statements (when you commit the changes). – Deuteragonist