How to use Union with different type of collections?

Question

My method will need to return a collection that contains settings.AgentIds and agent ids that correspond to settings.LabelIds. So I decided to use Union

IEnumerable<IAgentsGroup> labelGroups = _agentsGroups.Values.Where(x => settings.LabelIds.Contains(x.Id));

IEnumerable<IEnumerable<Guid>> labelAgentIds = labelGroups.Select(x => x.AgentIds);

return labelAgentIds.Union(settings.AgentIds);

But I dont now how to combine it into the one Collection<Guid>,

beacuse settings.AgentIds has type Collection<Guid> and labelAgentIds has IEnumerable<IEnumerable<Guid>> type?

possible duplicate of [Flatten IEnumerable>; understanding generics](http://stackoverflow.com/questions/9440497/flatten-ienumerableienumerable-understanding-generics) — Razvan Dumitru, Sep 23 '15 at 11:54

Jon Skeet · Accepted Answer · 2015-09-23T11:54:19.773

If you're content to just flatten the IEnumerable<IEnumerable<Guid>> into an IEnumerable<Guid> in the obvious way, then SelectMany is your friend:

IEnumerable<Guid> labelAgentIds = labelGroups.SelectMany(x => x.AgentIds);
return labelAgentIds.Union(settings.AgentIds);

Whereas Select maps each single element in the input to another single element in the output, SelectMany maps each single element in the input to any number of elements in the output (maybe 0, maybe 1, maybe more).

score 1 · Answer 2 · answered Sep 23 '15 at 11:52

1

You can use SelectMany to flatten the collection:

IEnumerable<Guid> labelAgentIds = labelGroups.SelectMany(x => x.AgentIds);

answered Sep 23 '15 at 11:52

Haney

32,775
8
59
68

score 1 · Answer 3 · answered Sep 23 '15 at 11:54

1

I think you want to flatten the IEnumerable<Collection<Guid>>, then use SerlectMany:

var allGuids = labelAgentIds.SelectMany(col => col).Union(settings.AgentIds);
Collection<Guid> result = new Collection<Guid>(allGuids.ToList());

answered Sep 23 '15 at 11:54

Tim Schmelter

450,073
74
686
939

How to use Union with different type of collections?

3 Answers3