Going from IQueryable<IEnumerable<myObj>> to IEnumerable<myObj>

Question

I have a "GetPersonsPerDepartment()" method returning a IQueryable<IEnumerable<Person>> while I'm trying to get a simple List<Person> or IEnumerable<Person> with all the persons from every department. I wrote a simple method GetAllPersons() that solves this with a loop but I feel I must be missing a simple command to get the same result:

private List<Person> GetAllPersons()
{
    List<Person> allPersons = new List<Person>();

    foreach(var Persons in GetPersonsPerDepartment())
    {
        allPersons.AddRange(Persons);
    }

    return allPersons;
}

private IQueryable<IEnumerable<Person>> GetPersonsPerDepartment()
{
    IQueryable<IEnumerable<Person>> Persons;

    using (var context = MySource.GetPersonsContext())
    {
        var library = "Personlist";
        var PersonFolderList = context.Web.Lists.GetByTitle(library);
        context.Load<List>(PersonFolderList);

        var PersonFolderListItems = PersonFolderList.GetItems(new CamlQuery());
        context.Load<ListItemCollection>(PersonFolderListItems);

        context.ExecuteQuery();

        Persons = PersonFolderListItems.Where(x => x.FileSystemObjectType == FileSystemObjectType.Folder).Select(x => GetPersonsFromFolderListItem(x, context, PersonFolderList));
    }

    return Persons;
}

Answer 1

Use SelectMany. As in, allPersons.SelectMany(s => s);. This flattens the sequence for you.

Answer 2

Is this a bug or dirty workaround?

There is one thing in your code that sooner or later will bite you and I don't quite understnd why it didn't...

You're returing an IQueryable<IEnumerable<Person>> after you have disposed your context. I'm not sure what workarounds your are using there because you provided very little context but your code actually shouldn't work.

I suppose it's Entity Framework and with it you should never return IQuerybale if you're disposing its context by the same method but always materialize the query before returnig the result, e.g. with ToList etc.