Question

I am going to use some basic stripped down examples to illustrate my problem.

I have a class:

class Item
{
   int ID;
   bool Selected;
}

Now lets say I have two lists of the Item class:

List<Item> ListA = GetListA();
List<Item> ListB = GetListB();

Now I want to create a third list that has all the items from ListB. The important thing is that if a match (same ID) if found in ListA then I want to use that Selected value, otherwise I want to keep the Selected value of the item that is in ListB.

I am creating the third list as follows:

List<Item> ListC = from item in ListB 
select new Item
{
   ID = item.ID,
   Selected = item.Selected// <-- should use value form ListA if available
};

Important: I don’t want to seem ignorant, but I do not want to change the way ListC is created. By that I mean I want to use the “linq select” method, and I want to use a “one liner” that assigns the Selected value… I know there are other ways to create the list which will work just fine, but then I won’t learn anything new.

---

I have tried a couple of things so far…

I know this will work, but I don’t want to query ListA twice:

Selected = ListA.Any(x => x.ID == item.ID) ? ListA.First(x => x.ID == item.ID).Selected : item.Selected

and I also tried using DeafultIfEmpty but I don’t think that is the right think for this situation… because it didn’t work, and it seems it is more use if ListA was empty (which I don’t care about)

Answer 1

You can modify your last line of code as follows:

Selected = (ListA.FirstOrDefault(x => x.ID == item.ID) ?? item).Selected;

For DefaultIfEmpty, you’d have to do

Selected = ListA.Where(x => x.ID == item.ID).DefaultIfEmpty(item)
                                                       .First().Selected;

which is harder to follow but does essentially the same thing.

As Dominic has noted below, if you use SingleOrDefault/Single in place of FirstOrDefault/First, you will get an exception if you find two items in ListA with the same ID – which may be a check you want to introduce.

(There are better/more efficient ways of doing this, but as you say, you want to fix this way of doing it, not use a totally different way.)