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c# IEnumerable<Object> LINQ GroupBy - Merge 2 Groups

I'm working on some LINQ GroupBy logic and I can't think how to elegantly and efficiently get this to work.
Basically, I have an IEnumerable<Thing> object (which is in the correct order!), where each Thing object has a RootId property. I want to group these objects on their RootId, which I have working:
IEnumerable<Thing> things; // Already has value assigned
var groups =
(from thing in things
group thing by thing.RootId into thingGroup
select thingGroup.ToArray())
.ToList();
groups is of type List<Thing[]>
Now here is the problem!
The above example is returning 5 items in the list. But, how would I merge 2 of the arrays into 1, leaving 4 items (again, keeping the order of course)??
The reason why is because 2 of the items has different RootId's but I want them to be treated the same i.e. grouped together.
I was going to concat and manipulate the arrays after the LINQ statement, but really it needs to be done as part of the group by/LINQ - any ideas?
Let me know if further examples or information is needed.
Thanks!
The merging criteria will be a manual process, so I was thinking of passing it into the groupby method like so:
var rootIdsToMerge = new List<Tuple<ID, ID>>
{
new Tuple<ID, ID>(rootIdOne, rootIdTwo),
new Tuple<ID, ID>(rootIdThree, rootIdFour)
};
So any group item with a RootId of rootIdOne will be merged with the group item with a RootId of rootIdTwo, and so on.

Since you are not using the grouping Key, you can associate the Item2 from the mapping to Item1 as a RootId key to group by:
var groups =
(from thing in things
group thing by rootIdsToMerge.FirstOrDefault(e => e.Item2 == thing.RootId)?.Item1 ?? thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();
Or in pre C#6 (no .? operator):
var groups =
(from thing in things
let mergeWith = rootIdsToMerge.FirstOrDefault(e => e.Item2 == thing.RootId)
group thing by mergeWith != null ? mergeWith.Item1 : thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();
Update: If you just want to consolidate a list of RootIds, then you can use a combination of Contains and First:
List<ID> rootIdsToMerge = ...;
var groups =
(from thing in things
group thing by rootIdsToMerge.Contains(thing.RootId) ? rootIdsToMerge.First() : thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();
The variants with
List<List<ID>> rootIdsToMerge = ...;
are similar to the initial variant with tuples:
var groups =
(from thing in things
group thing by rootIdsToMerge.FirstOrDefault(ids => ids.Contains(thing.RootId))?.First() ?? thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();
or
var groups =
(from thing in things
let mergeList = rootIdsToMerge.FirstOrDefault(ids => ids.Contains(thing.RootId))
group thing by mergeList != null ? mergeList.First() : thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();

LINQ Intersect on inner collection

I have a list of Stores (of type ObservableCollection<Store>) and the Store object has a property called Features ( of type List<Feature> ). and the Feature object has a Name property (of type string).
To recap, a list of Stores that has a list of Features
I have a second collection of DesiredFeatures (of type List<string> ).
I need to use LINQ to give me results of only the stores that have all the DesiredFeatures. So far, I've only been able to come up with a query that gives me an OR result instead of AND.
Here's what that looks like:
var q = Stores.Where(s=> s.Features.Any(f=> DesiredFeatures.Contains(f.name)));
I know Intersect can help, and here's how I've used it:
var q = Stores.Where(s => s.Features.Intersect<Feature>(DesiredFeatures));
This is where I'm stuck, Intersect wants a Feature object, what I need to intersect is on the Feature.Name.
The goal is to end up with an ObservableCollection where each Store has all of the DesiredFeatures.
Thank you!

You've almost done what you need. A small refine would be to swap DesiredFeatures and s.Features.
var q = Stores.Where(s => DesiredFeatures.All(df => s.Features.Contains(df)));
It means take only those stores where desired features are all contained in features of the store.

I need to use LINQ to give me results of only the stores that have all the DesiredFeatures.
In other words, each desired feature must have a matching store feature.
I don't see how Intersect can help in this case. The direct translation of the above criteria to LINQ is like this:
var q = Stores.Where(s =>
DesiredFeatures.All(df => s.Features.Any(f => f.Name == df))
);
A more efficient way could be to use a GroupJoin for performing the match:
var q = Stores.Where(s =>
DesiredFeatures.GroupJoin(s.Features,
df => df, sf => sf.Name, (df, sf) => sf.Any()
).All(match => match)
);
or Except to check for unmatched items:
var q = Stores.Where(s =>
!DesiredFeatures.Except(s.Features.Select(sf => sf.Name)).Any()
);

Going on your intersect idea, the only way I thought of making this work was by using Select to get the Store.Features (List<Feature>) as a list of Feature Names (List<string>) and intersect that with DesiredFeatures.
Updated Answer:
var q = Stores.Where(s => s.Features.Select(f => f.Name).Intersect(DesiredFeatures).Any());
or
var q = Stores.Where(s => DesiredFeatures.Intersect(s.Features.Select(f => f.Name)).Any());
Old Answer (if DesiredFeatures is a List<Feature>):
var q = Stores.Where(s => s.Features.Select(f => f.Name).Intersect(DesiredFeatures.Select(df => df.Name)).Any());

Two things you want your code to perform.
var q = Stores.Where(s=> s.Features.All(f=> DesiredFeatures.Contains(f.name)) &&
s.Features.Count() == DesiredFeatures.Count()); // Incude Distinct in the comparison if Features list is not unique
Ensure that every Feature is DesiredFeature
Store contains all Desired features.
Code above assumes uniqueness in Features collection as well as DesiredFeatures, modify code as stated in comment line if this is not right

Select 1 column from a Group By LINQ query

I think what I need is relatively simple but every example I Google just returns results using First(), which I'm already doing. Here is my expression:
var options = configData.AsEnumerable().GroupBy(row => row["myColumn"]).Select(grp => grp.First());
What I need is only ONE column from the grp portion and to be able to suffix .ToList() on there without an error. As it stands I receive 4 columns, but only need a specific one, kind of like if this (grp => grp["myColumn"]), didn't result in error the Error 153 Cannot apply indexing with [] to an expression of type 'System.Linq.IGrouping<object,System.Data.DataRow>'
Also, Key does not work in the grouping portion as these results are from a DataTable object. See here - >

If you want only the keys, you can use
var options = configData.AsEnumerable().Select(row=>row["myColumn"]).Distinct();

I think that this is what you want:
configData.AsEnumerable()
.GroupBy(r => r["myColumn"])
.Select(g => new
{
myColumnValue = g.Key,
myColumnItems = g.Select(r => r["OtherColumn"]).ToList()
});
Do you understand how/what this does though? Try it out and inspect the resulting IEnumerable. I'm not sure you have a perfect understanding on how GroupBy works but take your time with above example.
See this part:
new
{
myColumnValue = g.Key,
myColumnItems = g.Select(r => r["OtherColumn"]).ToList()
}
This creates an anonymous type which outputs the values of "OtherColumn" column into a list grouped by "myColumn" where value of "myColumn" is in the myColumnValue property.
I'm not sure this answers your question but it looks like this is what you want.
The variable g is of the type IGrouping<object, DataRow>, it's not DataRow. The IGrouping interface is designed to provide a list of DataRow's grouped by object values - it does not produce a flat list, if it did then it would just be a Sort, not GroupBy.

Just specify the field you want after your call to First() e.g.
.Select(grp => grp.FirstOrDefault()["MyFieldName"]);
This will take the first record from the grouping and select the specified field from that record.

Lambda or LINQ for Complex Filter?

I have a need to filter a large collection of objects (in memory) to select only those which meet ALL of the selected categories.
This is essentially the SQL query I'm looking to replicate, but I've been unable to come up with a good C# alternative:
select distinct o.ObjectId
from Object o
join ObjectCategories oc on oc.ObjectId = o.ObjectId
where oc.CategoryId in (1)
and oc.CategoryId in (2)
and oc.CategoryId in (3)
... and so on...
...where 1, 2, and 3 represent the values in an indeterminate number of user-selected categories.

Have your user selected category ID's in a List and then you can use Contains.
select distinct o.ObjectId
from Object o
join ObjectCategories oc on oc.ObjectId = o.ObjectId
where yourCategoryList.Contains(oc=>oc.categoryID);

var results = ObjectCategories.Where(t2 => ObjectList.Any(t1 => t2.Contains(t1)) == true)
you can count the number of matches and if it is equal to the list you are checking against, then you have all the matches.

Consider using Dynamic LINQ. It allows you to use string expressions instead of lambda expressions. You should be able to do what you want using something similar to:
var qry = ObjectCategories.Where(
"ObjectList.CategoryId.Contains(1) AND ObjectList.CategoryId.Contains(2) ..."
);
There is a pretty solid implemention of Dynamic LINQ here: https://github.com/NArnott/System.Linq.Dynamic

Get ICollection out from IQueryable<ICollection> LINQ Query

I'm trying to write a query that grabs a list of countries out from my joined data.
Places is List<Country>.
var zonedCountries = (from dz in db.DeliveryZones.Include(d => d.Places)
where model.DeliveryZones.Contains(dz.ID)
select dz.Places);
I would expect zonedCountries to be a List but instead it is a IQueryable<ICollection<Country>>.
How do I extract the list from this?

If you want to get flattened list of countries:
var zonedCountries = (from dz in db.DeliveryZones.Include(d => d.Places)
where model.DeliveryZones.Contains(dz.ID)
from p in dz.Places
select p);
Or use SelectMany:
var zonedCountries = db.DeliveryZones.Include(d => d.Places)
.Where(dz => model.DeliveryZones.Contains(dz.ID))
.SelectMany(dz => dz.Places);
BTW I'm not sure if you need to include places manually in this case (thus you are selecting places instead of delivery zones). And you will probably want to select distinct countries only - Distinct() will help you here. Also if you want to store results in list, then simple ToList() call will do the job.