Linq list of lists to single list

Question

Seems like this is the kind of thing that would have already been answered but I'm unable to find it.

My question is pretty simple, how can I do this in one statement so that instead of having to new the empty list and then aggregate in the next line, that I can have a single linq statement that outputs my final list. details is a list of items that each contain a list of residences, I just want all of the residences in a flat list.

var residences = new List<DAL.AppForm_Residences>();
details.Select(d => d.AppForm_Residences).ToList().ForEach(d => residences.AddRange(d));

Answer 1

Use SelectMany

var all = residences.SelectMany(x => x.AppForm_Residences);

Answer 2

You want to use the SelectMany extension method.

var residences = details.SelectMany(d => d.AppForm_Residences).ToList();