Group by in LINQ with 2 attributes

Question

This is an already asked question, but that question is to work with 2 attributes only, I need to work with 3 attributes, So I am copy-pasting most of the text.

Let's suppose if we have a class like

class Person { internal int PersonID; internal string car ; internal string friend ; }

Now I have a list of this class: List persons;

Now this list can have instances multiple same PersonIDs, for ex.

persons[0] = new Person { PersonID = 1, car = "Ferrari" , friend = "Josh" }; persons[1] = new Person { PersonID = 1, car = "BMW" , friend = "Olof" }; persons[2] = new Person { PersonID = 2, car = "Audi" , friend = "Gustaf" };

Is there a way I can group by PersonID and get the list of all the cars and friends he has? For example the expected result would be:

class Result { int PersonID; List<string> cars; List<string> friends; }

From what I have done so far:

IEnumerable resultsForDisplay = ResultFromSQL_Query.GroupBy( p => p.PersonId.ToString(), p => p.car, (key, g) => new { PersonId = key, car = g.ToList()});

But now I'm stuck at getting the friend's array in resultsForDisplay

willeM_ Van Onsem · Accepted Answer · 2017-06-15 00:31:37Z

5

Sure, you can perform LINQ queries on the group g as well, like:

IEnumerable<Result> resultsForDisplay = from q in ResultFromSQL_Query group q by q.PersonID into g select new Result {PersonID = g.Key,cars = g.Select(x => x.car).ToList(), friends = g.Select(x => x.friend).ToList()};

Or with lambda expressions:

IEnumerable<Result> results = persons.GroupBy(x => x.PersonID) .Select(g => new Result { PersonID = g.Key, cars = g.Select(x => x.car).ToList(), friends = g.Select(x => x.friend).ToList()};

So you can perform any LINQ query on a group (which thus behaves as an IEnumerable<> on the elements of that grou), like a .Select(..), but also .Sum(..), .Average(..) and other sub queries, aggregates, etc.

edited Jun 15, 2017 at 0:31

answered Jun 14, 2017 at 23:56

willeM_ Van Onsem

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5 Comments

Atif Waqar Over a year ago

Hi, please forgive me, but im a newbie in Linq and Lambda expression. Could you please convert this syntax to lambda expression?

Atif Waqar Over a year ago

Also, when i try to use your syntax, it gives me an error on Result in line select new Result { the error says The type or namespace name 'Result' could not be found (are you missing a using directive or an assembly reference?)

willeM_ Van Onsem Over a year ago

@AtifWaqar: yes, these have to be written on a single line I think.

Chris Over a year ago

@AtifWaqar: Result is a class in your question. If you haven't got a proper reference to it then only you can answer why that is. Are you missing a using directive perhaps? Or did you just not put the class in your code and just in the question?

Atif Waqar Over a year ago

@WillemVanOnsem This is Amazing. I was stuck at this for a long time. Thanks you very much for the solution :) About Result, I got it, its the returning data type.

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