how do I treat null lists like empty lists in linq?

Question

Below is some linqpad test code. When this runs it errors because the second instance of "item" has a null list of subitems as opposed to an empty list.

I want to treat both situations (null or empty list) in exactly the same way but I wondered if there was a cleaner way than just putting a null check on the list and initialising an empty list when there's a null.

in other words, I could do this:

from si in (i.subitems == null ? new List<item>() : i.subitems)

but that's a little ugly and I wondered how I could improve on that?

public class item
{
    public string itemname { get; set; }
    public List<item> subitems { get; set; }
}

void Main()
{
    List<item> myItemList = new List<item>() 
    {
        new item 
        {
            itemname = "item1",
            subitems = new List<item>()
            {
                new item { itemname = "subitem1" },
                new item { itemname = "subitem2" }
            }
        },
        new item 
        {
            itemname = "item2"
        }
    };

    myItemList.Dump();

    var res = (from i in myItemList
            from si in i.subitems
            select new {i.itemname, subitemname = si.itemname}).ToList();

    res.Dump();
}

as a bonus question, can this same linq query be represented as a lambda and treat nulls the same way?

Cheers, Chris

Answer 1

You could use the null coalescing operator

var res = (from i in myItemList
           from si in i.subitems ?? new List<item>()
           select new { i.itemname, subitemname = si.itemname }).ToList();

But I think you should just filter the empty ones out

var res = (from i in myItemList
           where i.subitems != null
           from si in i.subitems
           select new { i.itemname, subitemname = si.itemname }).ToList();

As for a lambda version you could say

var res = myItemList.Where(x => x.subitems != null)
                    .SelectMany(
                        x => x.subitems.Select(
                            y => new { x.itemname, subitemname = y.itemname }
                        )
                     );

But the query syntax version is way more readble.