Linq Select: Using a value from a list if available

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 ListA = GetListA(); List 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 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 wo…

Accepted Answer

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.)

Linq Select: Using a value from a list if available

Solution

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