How to flatten list of lists?

Question

I have a query, which has one to many relation with LegalEntity table, my goal is to get a List of all LegalEntityIds. At this point I have following select which produces List<IEnumerable<string>>, is there a quick and efficient way without looping and creating separate list to flatten the inner lists into one.

var legalEntityIds = query.Select(x => x.LegalEntities.Select(y => y.LegalEntityId)).ToList();

score 7 · Accepted Answer · answered Feb 02 '22 at 09:06

Use SelectMany:

var legalEntityIds = 
    query.SelectMany(x => x.LegalEntities).Select(y => y.LegalEntityId).ToList();

or, using query syntax:

var legalEntityIds = (
    from item in query
    from legalEntity in item
    select legalEntity.LegalEntityId
).ToList();

How to flatten list of lists?

1 Answers1