Develop Reference

Multiple Group By LINQ extension methods

I have a C#.NET application and want to do Group By on multiple conditions.
I have a list like this:
var testq = new List<TestQuestion>()
{
new TestQuestion
{
Id = 1,
QuestionId = 1,
SelectedAnswerId = null
},
new TestQuestion
{
Id = 2,
QuestionId = 2,
SelectedAnswerId = 1
},
new TestQuestion
{
Id =3,
QuestionId = 1,
SelectedAnswerId = 1
},
new TestQuestion
{
Id = 4,
QuestionId = 3,
SelectedAnswerId = 5
},
new TestQuestion
{
Id = 5,
QuestionId = 1,
SelectedAnswerId = 2
},
new TestQuestion
{
Id = 6,
QuestionId = 3,
SelectedAnswerId = 3
},
new TestQuestion
{
Id =7,
QuestionId = 4,
SelectedAnswerId = null
},
new TestQuestion
{
Id =8,
QuestionId = 5,
SelectedAnswerId = null
},
};
My code is :
var result = testq
.Where(p => p.SelectedAnswerId.HasValue)
.GroupBy(p => p.QuestionId)
.Select(p => p.FirstOrDefault())
.ToList();
now, result ID's is (2 ,3, 4)
but result is not true...
The result should be this :
ID's -> (2 ,3, 4, 7, 8)
I want to group by the result based on the QuestionID field and The first record that does not have a (SelectedAnswerId)field value is empty,
Also, records in which the question ID is only there once, regardless of the value of the field (SelectedAnswerId) in the output. that's mean, last two items in the list
please guide me...

Try this:
var result = testq
.Where(p => p.SelectedAnswerId.HasValue || testq.Count(x => x.QuestionId == p.QuestionId) == 1)
.GroupBy(p => p.QuestionId)
.Select(p => p.FirstOrDefault())
.Distinct()
.ToList();
C# Fiddle

You have to filter the SelectedAnswerId in Select not in the Where clause. Try the below,
var result = testq
.GroupBy(p => p.QuestionId)
.Select(p =>
{
var grouped = p.ToList(); //Get the groupBy list
TestQuestion testQuestion = grouped.FirstOrDefault(x => x.SelectedAnswerId.HasValue); //Get anything with AnswerId
return testQuestion ?? grouped.FirstOrDefault(); //If not not available with Answer then get the First or Default value
})
.ToList();
C# Fiddle with test data.

Although after a GroupBy the order of the elements in each group is fairly defined (see Enumerable.GroupBy, it seems that the element that you want is not the First one in each group.
You want the First element of each group that has a non-null SelectedAnswerId, or if there is no such one, you want the First element of each group that has a null SelectedAnswerId.
How about this:
var result = testQ.GroupBy(question => question.QuestionId);
// every group contains a sequence of questions with same questionId
// select the ones with a null SelectedAnswerId and the ones with a non-null value
.Select(group => new
{
NullSelectedAnswer = group
.Where(group.SelectedAnswerId == null)
.FirstOrDefault(),
NonNullselectedAnswer = group
.Where(group.SelectedAnswerId != null)
.FirstOrDefault(),
})
// if there is any NonNullSelectedAnswer, take that one, otherwise take the null one:
.Select(selectionResult => selectionResult.NonNullSelectedAnswer ??
selectionResult.NullSelectedAnswer);

How to group and merge/flatten a list of anonymous objects in LINQ

I have a list of anonymous objects generated by a LINQ query that I do not have access to modify.
The objects have the following properties:
OrderId, RepId, FirstName, LastName, Address
Each "Rep" often places multiple orders, so there are a lot of rows where the only difference is the OrderId. There is a requirement that if the same Rep has placed multiple orders, to batch these together in groups of 6 with a new structure:
OrderId1, OrderId2, ..., OrderId6, RepId, FirstName, LastName, Address
But if the rep has placed say 8 orders, there would be a batch of 6 and a batch of 2. So the new objects don't always have the same number of properties.
I've started by grouping the initial result set by RepId, but I have no clue where to go next.
Is this possible using LINQ?

As your output have anonymous objects with different schema, that make the thing a little more complicate.
Ideally you should design your entity class to use list for orders instead of property like "OrderId1", "OrderId2"... That is not extensible and error prone. But for that specific question, we can combine LINQ and ExpandoObject to achieve this.
orders.GroupBy(order => order.RepId)
.SelectMany(orderGroup => orderGroup.Select((order, i) => new {
Order = order,
ReqId = orderGroup.Key,
SubGroupId = i / 6
}))
.GroupBy(h => new {
ReqId = h.ReqId,
SubGroupId = h.SubGroupId,
FirstName = h.Order.FirstName,
LastName = h.Order.LastName,
Address = h.Order.Address
})
.Select(orderWithRichInfo => {
dynamic dynamicObject = new ExpandoObject();
int i = 1;
foreach(var o in orderWithRichInfo)
{
((IDictionary<string, object>)dynamicObject).Add("OrderId" + i, o.Order.OrderId);
i++;
}
((IDictionary<string, object>)dynamicObject).Add("FirstName", orderWithRichInfo.Key.FirstName);
((IDictionary<string, object>)dynamicObject).Add("LastName", orderWithRichInfo.Key.LastName);
((IDictionary<string, object>)dynamicObject).Add("Address", orderWithRichInfo.Key.Address);
return dynamicObject;
});
Hope it helps.

First option.
If you want to get 6 OrderId-s as a list, you can create
class OrderBundle
{
public int RepId { get; set; }
public List<int> OrderIds { get; set; }
}
Group your items:
var orderBundels = orderList
.GroupBy(m => m.RepId)
.Select(g => new OrderBundle
{
RepId = g.Key,
OrderIds = g.Select(m => m.OrderId).ToList()
});
And then split them into groups:
List<OrderBundle> dividedOrderBundels = new List<OrderBundle>();
foreach (OrderBundle orderBundle in orderBundels)
{
int bundelCount = (int)Math.Ceiling(orderBundle.OrderIds.Count() / 6.0);
for (int i = 0; i < bundelCount; i++)
{
OrderBundle divided = new OrderBundle
{
RepId = orderBundle.RepId,
OrderIds = orderBundle.OrderIds.Skip(i * 6).Take(6).ToList()
};
dividedOrderBundels.Add(divided);
}
}
Second option:
You can achieve the same result without creating model like below:
var result = orderList
.GroupBy(m => m.RepId)
.SelectMany(g => g.Select((m, i) => new
{
RepId = g.Key,
FirstName = m.FirstName,
LastName = m.LastName,
Address = m.Address,
OrderId = m.OrderId,
BunleIndex = i / 6
}))
.GroupBy(m => m.BunleIndex)
.Select(g => new
{
RepId = g.Select(m => m.RepId).First(),
FirstName = g.Select(m => m.FirstName).First(),
LastName = g.Select(m => m.LastName).First(),
Address = g.Select(m => m.Address).First(),
OrderIds = g.Select(m => m.OrderId).ToList()
})
.ToList()

Linq query to merge distinct items in a list [duplicate]

This question already has an answer here:
Grouping a generic list via LINQ in VB.NET
(1 answer)
Closed 7 years ago.
I have the following model:
public class Result
{
public int Id { get; set; }
public string Company { get; set; }
}
And I have a List with data similar to the following:
Id Company
=================
21 Microsoft
22 Apple
22 IBM
23 Microsoft
How can I use Linq to give me the distinct ID's, concatenating the Company column with a delimiter?
My output should be:
Id Company
=================
21 Microsoft
22 Apple, IBM
23 Microsoft

You can use GroupBy and String.Join:
IEnumerable<Result> query = results.GroupBy(r => r.Id)
.Select(g => new Result
{
Id = g.Key,
Company = String.Join(", ", g.Select(r => r.Company))
});

A slightly different take on Tim's excellent answer if there are duplicate records in your source and you don't want Company names repeated in the same Field:
var data = new List<Result>
{
new Result {Id = 21, Company = "Microsoft"},
new Result {Id = 22, Company = "Apple"},
new Result {Id = 22, Company = "IBM"},
new Result {Id = 23, Company = "Microsoft"},
new Result {Id = 23, Company = "Microsoft"}
};
var x = data.GroupBy(d => d.Id)
.Select(d => new Result { Id = d.Key,
Company =
string.Join(",", d.Select(s => s.Company).Distinct())});

var groupesList = result.GroupBy(x => x.Id,
(key, val) => new { Key = key, Value = string.Join(",", val.Select(r => r.Company)} ).ToList();
then you can call Key(unuque) or Value by key for all inform for example all ID

Simply use GroupBy and String.Join methods:-
List<Result> result = Results.GroupBy(x => x.id)
.Select(x => new Result
{
Id = x.Key,
Company = String.Join(",",x.Select(z => z.Company)
}).ToList();

How do I get a distinct list of list types

This is my code (copy and paste it into linqpad if you like)
var messageUsers = new [] {
new { MsgId = 2, UserId = 7 },
new { MsgId = 2, UserId = 8 },
new { MsgId = 3, UserId = 7 },
new { MsgId = 3, UserId = 8 },
new { MsgId = 1, UserId = 7 },
new { MsgId = 1, UserId = 8 },
new { MsgId = 1, UserId = 9 }};
messageUsers
.GroupBy (x => x.MsgId, x => x.UserId)
.Select (x => x.Select (y => y))
.Distinct()
.Dump();
The results I get back are {7,8}, {7,8}, {7,8,9}
What I want is {7,8}, {7,8,9}.
Basically I want to remove the duplicate lists.
I haven't tried this but I think I could probably achieve by creating a comparer and passing it into the Distinct method. However I would like to eventually use this in a Linq to Entities query without bringing thousands of rows back to the client so that isn't a good option.
For extra clarification...I need to return a List> where the contents of each inner list is distinct in comparison to any of the other inner list.

The problem is that .Distinct() determines what's distinct based on the GetHashCode() and Equals() implementation of the underlying objects. In this case, the underlying object is something that implements IEnumerable<>, but which uses the default object implementation for those methods--which is based purely on whether the objects occupy the same space in memory. So as far as it can tell, the sequences are not distinct, even though they have the same values in them.
How about this?
messageUsers
.GroupBy (x => x.MsgId, x => x.UserId)
.GroupBy(x => string.Join(",", x))
.Select(x => x.FirstOrDefault())
.Dump();
The idea is to group by a key that represents the combined value of the elements in your list. You could also pass a custom IEqualityComparer<> to the Distinct method in your original code, but that seems like a fair bit of effort for something so trivial.
It's worth noting that this won't work very well if you're using LINQ to Entities or something like that.
Update
To make it a List<List<int>>, you'll need a few .ToList()s thrown in there:
messageUsers
.GroupBy (x => x.MsgId, x => x.UserId)
.GroupBy(x => string.Join(",", x))
.Select(x => x.FirstOrDefault().ToList())
.ToList()
.Dump();
But I'm frankly not sure why that matters to you.

Here is an alternative answer:
messageUsers
.GroupBy (x => x.MsgId, y=>y.UserId)
.Select (x => new HashSet<int>(x))
.Distinct(HashSet<int>.CreateSetComparer())
.Dump();
Consider the following input:
var messageUsers = new [] {
new { MsgId = 2, UserId = 7 },
new { MsgId = 2, UserId = 8 },
new { MsgId = 3, UserId = 8 },
new { MsgId = 3, UserId = 7 },
new { MsgId = 1, UserId = 7 },
new { MsgId = 1, UserId = 8 },
new { MsgId = 1, UserId = 9 }};
What result do you want?
{7,8}, {7,8,9} or {7,8}, {8,7}, {7,8,9}.

how to get an ordered list with default values using linq

I have an ICollection of records (userID,itemID,rating) and an IEnumerable items
for a specific userID and each itemID from a set of itemIDs, i need to produce a list of the users rating for the items or 0 if no such record exists. the list should be ordered by the items.
example:
records = [(1,1,2),(1,2,3),(2,3,1)]
items = [3,1]
userID = 1
result = [0,2]
my attempt:
dataset.Where((x) => (x.userID == uID) & items.Contains(x.iID)).Select((x) => x.rating);
it does the job but it doesn't return 0 as default value and it isnt ordered...
i'm new to C# and LINQ, a pointer in the correct direction will be very appreciated.
Thank you.

This does the job:
var records = new int[][] { new int[] { 1, 1, 2 }, new int[] { 1, 2, 3 }, new int[] { 2, 3, 1 } };
var items = new int[] { 3, 1 };
var userId = 1;
var result = items.Select(i =>
{
// When there's a match
if (records.Any(r => r[0] == userId && r[1] == i))
{
// Return all numbers
return records.Where(r => r[0] == userId && r[1] == i).Select(r => r[2]);
}
else
{
// Just return 0
return new int[] { 0 };
}
}).SelectMany(r => r); // flatten the int[][] to int[]
// output
result.ToList().ForEach(i => Console.Write("{0} ", i));
Console.ReadKey(true);

How about:
dataset.Where((x) => (x.userID == uID)).Select((x) => items.Contains(x.iID) ? x.rating : 0)

This does the job. But whether it's maintainable/readable solution is topic for another discussion:
// using your example as pseudo-code input
var records = [(1,1,2),(1,2,3),(2,3,1)];
var items = [3,1];
var userID = 1;
var output = items
.OrderByDescending(i => i)
.GroupJoin(records,
i => i,
r => r.ItemId,
(i, r) => new { ItemId = i, Records = r})
.Select(g => g.Records.FirstOrDefault(r => r.UserId == userId))
.Select(r => r == null ? 0 : r.Rating);
How this query works...
ordering is obvious
the ugly GroupJoin - it joins every element from items with all records that share same ItemId into annonymous type {ItemId, Records}
now we select first record for each entry that matches userId - if none is found, null will be returned (thanks to FirstOrDefault)
last thing we do is check whether we have value (we select Rating) or not - 0

How about this. your question sounds bit like an outer join from SQL, and you can do this with a GroupJoin, SelectMany:
var record1 = new Record() { userID = 1, itemID = 1, rating = 2 };
var record2 = new Record() { userID = 1, itemID = 2, rating = 3 };
var record3 = new Record() { userID = 2, itemID = 3, rating = 1 };
var records = new List<Record> { record1, record2, record3 };
int userID = 1;
var items = new List<int> { 3, 1 };
var results = items
.GroupJoin( records.Where(r => r.userID == userID), item => item, record => record.itemID, (item, record) => new { item, ratings = record.Select(r => r.rating) } )
.OrderBy( itemRating => itemRating.item)
.SelectMany( itemRating => itemRating.ratings.DefaultIfEmpty(), (itemRating, rating) => rating);
To explain what is going on
For each item GroupJoin gets the list of rating (or empty list if no rating) for the specified user
OrderBy is obvious
SelectMany flattens the ratings lists, providing a zero if the ratings list is empty (by DefaultIfEmpty)
Hope this makes sense.
Be aware, if there is more than one rating for an item by a user, they will all appear in the final list.