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Linq Flatten List of List [duplicate]

This question already has answers here:
Flatten a list which one of its properties is another list of object
(3 answers)
Closed 1 year ago.
Using Linq what will be the best way of making a flattened list with these 2 objects lists
public class DataDto
{
public string StudentID { get; set; }
public List<ScoreDto> Scores { get; set; }
}
public class ScoreDto
{
public string ExamID { get; set; }
public double Mark { get; set; }
}
into a list that will present as
public class FinalDto
{
public string StudentID { get; set; }
public string ExamID { get; set; }
public double Mark { get; set; }
}
With StudentID repetitions for the number of ScoreDtos present for a particular student.
Example data -
var data = new List<DataDto>()
{
new DataDto
{
StudentID = "S1",
Scores = new List<ScoreDto>()
{
new ScoreDto { ExamID = "01", Mark = 5},
new ScoreDto { ExamID = "02", Mark = 15},
new ScoreDto { ExamID = "03", Mark = 25}
}
},
new DataDto
{
StudentID = "S2",
Scores = new List<ScoreDto>()
{
new ScoreDto { ExamID = "01", Mark = 1},
new ScoreDto { ExamID = "02", Mark = 5},
new ScoreDto { ExamID = "03", Mark = 20}
}
}
};
So it produces -
StudentID, ExamID, Mark
S1, 01, 5
S1, 02, 15
S1, 03, 25
S2, 01, 1
S2, 02, 5
S2, 03, 20

You use SelectMany to flatten a list of lists:
var final = data.SelectMany(
student => student.Scores,
(student, score) => new FinalDto {
StudentID = student.StudentID,
ExamID = score.ExamID,
Mark = score.Mark
}
);
Console.WriteLine("StudentID, ExamID, Mark");
foreach (var result in final)
{
Console.WriteLine("{0}, {1}, {2}", result.StudentID, result.ExamID, result.Mark);
}
Alternatively, you can use a different overload of SelectMany along with a nested Select projection:
var final = data.SelectMany(
student => student.Scores.Select(
score => new FinalDto {
StudentID = student.StudentID,
ExamID = score.ExamID,
Mark = score.Mark
}
)
);
You can also use query syntax:
var final = (
from student in data
from score in student.Scores
select new FinalDto {
StudentID = student.StudentID,
ExamID = score.ExamID,
Mark = score.Mark
}
);
Note that this is just translated to the second form shown above.
There's no "best" here. Forms two and three literally compile to the same exact code. Form one is just a slight variation. There is no impact (speed, memory) for choosing one over the other. Pick the one that is visually appealing to you. Personally I don't write query syntax (the last form) and I always forget the first overload exists so I end up using the second one.

var result = (
from item in data
from score in item.Scores
select new FinalDto {
StudentID = item.StudentID,
ExamID = score.ExamID,
Mark = score.Mark
}).ToList();

You have two tables with a one-to-many relation: every item of table A has zero or more items from table B, and every item from table B belongs to exactly one item of table A, namely the one item that the foreign key refers to.
Whenever you have this, and you want "items with their sub-items", like Schools with their Students, Customers with their Orders and Authors with their Books, consider to use one of the overloads of Queryable.GroupJoin
If you don't want the items with their sub-items, but a flat table,consider to use Queryable.Join
Apparently you already have fetched the "Students with their zero or more Scores", and you want a flat result. In that case you use SelectMany
IEnumerable<DataDto> studentsWithTheirScores = ...
var result = studentsWithTheirScores.SelectMany(
// parameter collectionSelector: where are the sub-items stored?
student => student.Scores,
// parameter resultSelector: take each student, and each one of his Scores
// to make one new
(student, score) => new FinalDto
{
StudentId = student.StudentId,
ExamId = score.ExamId,
Mark = score.Mark,
});

C#: find objects within array with same id and sum their quantity values. Add those to the new object

I have an array or list of objects returned from the Database. Let's take this as an example:
this is the class:
public class products
{
public string name { get; set; }
public int quantity { get; set; }
public long id{ get; set; }
}
List<Product> prod = new List<Product>()
prod = (call to the DB to get data back...)
array is returned with object of products
What I need is to loop through this array and add up the quantities for the same ids. Meaning, add up 2+7 for id 3 and add that data to another object so that new object would have something like: total: 9, id: 3
then same again for id 5, total: 7, id: 5
and so on.
I am at a loss of the right way to do this. I was looking at Linq but the only way I used it is by providing specific values. Please help me with this
`
foreach (var p in prod){ Now what do i do?}
`

The easiest way is with GroupBy and Sum (Both use System.Linq):
List<Product> products = new List<Product>()
{
new Product(){Id = 1, Cost = 20.0M },
new Product(){Id = 1, Cost = 30.0M },
new Product(){Id = 2, Cost = 20.0M },
new Product(){Id = 3, Cost = 20.0M },
new Product(){Id = 3, Cost = 5.0M }
};
products.GroupBy(g => g.Id).ToList().ForEach(e =>
{
Console.WriteLine($"Total: {
e.Sum(s => s.Cost)} for {e.Key}");
});
Edit
With the new information provided: You can do this to a concrete class:
public class Product
{
public string name { get; set; }
public int quantity { get; set; }
public long id{ get; set; }
}
List<Product> products = new List<Product>();
var products = (call to the DB to get data back...).GroupBy(g => g.Id).ToList().ForEach(e =>
{
products.Add(new Product()
{
Id = e.Key,
Quantity = e.Sum(s => s.Quantity)
})
});

Per your code snippet, prod is single product of type products.
So, assuming your code which invokes database call returns prod list something like below:
List<products> prod = new List<products>();
prod = _productRepository.GetProductData(prodId);
You can use linq GroupBy (please include System.Linq to use GroupBy linq extension method) to find the total quantity of each product like below:
var anonymousList = prod.GroupBy(p => p.id).Select(g => new {id = g.Key, totalQuantity = g.Sum(p => p.quantity)}).ToList()
The above returns anonymous list of objects where each object contains id and totalQuantity.
If you are interested in dictionary of product id vs totalQuantity, then use something like below:
Dictionary<long, int> dictionary = prod.GroupBy(p => p.id).ToDictionary(k => k.Key, v => v.Sum(p => p.quantity));
UPDATE based on comments discussion:
You can invoke GroupBy on prod without verifying the count. No exception will be thrown.

Grouping Deeply Nested Objects Using LINQ

I have the following classes:
public class Item
{
public int Id { get; set; }
public string Sku { get; set; }
public List<Price> Prices { get; set; }
}
public class Price
{
public int Id { get; set; }
public double Cost { get; set; }
public PriceList List { get; set; }
}
public class PriceList
{
public int Id { get; set; }
public string FileName { get; set; }
public DateTime ImportDateTime { get; set; }
}
Essentially, it's for a list of items [Item] in which each item has a list of prices [List<Price> Prices] (one-to-many), and each price has one price list [PriceList List] (one-to-one).
What I need is a list of all price lists.
It seems like what I need is a group to a "grandchild", basically, grouping by the PriceList's Id (which is under Price, which in turn is under Item in the list).
So, as an example, if I have five price lists, it should return five rows.
I achieved it doing the following long way:
List<PriceList> priceLists = new List<PriceList>();
foreach (Item item in items)
{
foreach (Price price in item.Prices)
{
PriceList list = price.List;
if (!priceLists.Any(x => x.Id == list.Id))
{
priceLists.Add(list);
}
}
}
How can it be achieved using LINQ?
UPDATE:
Here's a basic sample set:
PriceList priceList1 = new PriceList { Id = 1, FileName = "Price List 1.csv" };
PriceList priceList2 = new PriceList { Id = 2, FileName = "Price List 2.csv" };
Price price1 = new Price { Id = 1, Cost = 2.65, List = priceList1 };
Price price2 = new Price { Id = 2, Cost = 14.23, List = priceList2 };
Price price3 = new Price { Id = 3, Cost = 29.01, List = priceList1 };
Price price4 = new Price { Id = 4, Cost = 1, List = priceList2 };
Price price5 = new Price { Id = 5, Cost = 56.12, List = priceList1 };
Item item1 = new Item { Id = 1, Sku = "item1", Prices = new List<Price> { price1, price2 } };
Item item2 = new Item { Id = 2, Sku = "item2", Prices = new List<Price> { price3, price4 } };
Item item3 = new Item { Id = 3, Sku = "item3", Prices = new List<Price> { price5 } };
List<Item> itemsList = new List<Item> { item1, item2, item3 };
Let's start from the bottom upwards:
I have a list of Item (itemsList), which contains three Items.
Each Item has a list of Price.
Each Price has one PriceList.
To clarify the reasoning behind all this: The user imports a spreadsheet of prices (price list) they get periodically from their supplier, prices fluctuate and each price list has different prices for the same items. Hence the reason of having an item with many prices and each price has its price list which was used to upload it (there's more information included in the PriceList class which I omitted to keep it simple).
I hope I'm clear.

This seems to be what you want, using an extension to do Distinct by a lambda expression:
var ans = itemsList.SelectMany(item => item.Prices.Select(price => price.List)).DistinctBy(price => price.Id);
The extension is as follows:
public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> src, Func<T, TKey> keyFun) {
var seenKeys = new HashSet<TKey>();
foreach (T e in src)
if (seenKeys.Add(keyFun(e)))
yield return e;
}
If you changed your approach to use a HashSet<Int> to track the seen ids you would probably be a tiny bit more efficient than LINQ.

It looks like you can combine a SelectMany (to get all the Prices lists) with a Select (to get all the associated PriceList properties):
List<PriceList> priceLists = items.SelectMany(i => i.Prices).Select(p => p.List).ToList();
Edit
The above will return all the PriceLists, but since many Items could reference the same PriceList, then you need a way to filter on the PriceList.Id field.
One way to do that would be to override the Equals and GetHashCode property of the PriceList object. If you truly consider two PriceList objects to be equal if they have the same Id, then you could do the following:
public class PriceList
{
public int Id { get; set; }
public string FileName { get; set; }
public DateTime ImportDateTime { get; set; }
public override bool Equals(object obj)
{
var other = obj as PriceList;
return Id == other?.Id;
}
public override int GetHashCode()
{
return Id;
}
}
This allows you to use the Linq method, Distinct on your prices (it will call the Equals method to distinguish between PriceLists in the results:
List<PriceList> priceLists = items
.SelectMany(i => i.Prices)
.Select(i => i.List)
.Distinct()
.ToList();

using linq to merge items together in a collection

I have a list of objects (call them type salesItems) Lets say these items have 50 properties, with Name, price, quantity being 3 of them). I would like to know how to merge the list together combining any salesItems by name using the following logic:
If there are multiple salesOrders that have the same Name:
Combine them into one SalesOrder with the same Name
Set the quantity to the sum of the quantities
Set the price, and all of the other properties using the values of the first
I would like to do with linq. I realize i could use a big for each c# loop instead.
If there are additional items in the list I would like to follow similar logic for those as well.
EX: A salesOrder list with (A,B,C,D,E)
A: Name=ball Price= 2.24 Quantity=1 (other values = bla bla)
B: Name= ball Price = 15.33 Quantity=3 (other values)
c: Name= bat Price = 22.14 Quantity=3 (other values)
D: Name= bat Price= 19.22 Quantity=2 (other values)
E: Name = ball Price=4.32 Quantity=2 (other values)
Result list I want 2 Sales orders in list (A,C) A: Name=ball Price=
2.24 Quantity=6 (other values = bla bla from a's properties) c: Name= bat Price = 22.14 Quantity=5 (other values from c's
properties)

You want linq's .GroupBy method!!!
I've defined your class as:
public class SalesOrder
{
public string Name { get; set; }
public double Price { get; set; }
public int Quantity { get; set; }
public SalesOrder(string Name, double Price, int Quantity)
{
this.Name = Name;
this.Price = Price;
this.Quantity = Quantity;
}
}
then I have created a list of your orders like this:
List<SalesOrder> Orders = new List<SalesOrder>()
{
new SalesOrder("Ball", 2.24, 1),
new SalesOrder("Ball", 15.33, 3),
new SalesOrder("Bat", 22.14, 3),
new SalesOrder("Bat", 19.22, 2),
new SalesOrder("Ball", 4.32, 2)
};
and grouped them by the name before selecting the values you want for each group into a new instance of the SalesOrder class like this:
List<SalesOrder> Combined_Orders = Orders
.GroupBy (o => o.Name)
.Select (o => new SalesOrder(o.Key, o.Select (x => x.Price).First(), o.Sum(x => x.Quantity)))
.ToList();
UPDATE: In response to OP's comment
As the real SalesOrder will have hundreds of properties, you can avoid typing them all out in the linq query by adding a constructor to the SalesOrder class that accepts the result of the group by as an argument, then do all the work in the constructor. While it doesn't stop you from having to type out all the properties, it does mean that its neatly abstracted away. Also this way it forces/enables you to decide on what to do with each of the properties (first/sum/average).
To do this you will need a second constructor that looks like this:
public SalesOrder(IGrouping<string, SalesOrder> Group)
{
this.Name = Group.Key;
this.Price = Group.First().Price;
this.Quantity = Group.Sum(g => g.Quantity);
// do all other properties here too
}
Then update the group by to look like this (note that only the result of the grouping "g" is passed into the constructor now):
List<SalesOrder> Combined_Orders = Orders
.GroupBy (o => o.Name)
.Select (g => new SalesOrder(g))
.ToList();

Hi You can use the following code,
class SalesItem
{
public string Name { get; set; }
public int Price { get; set; }
public int Quantity { get; set; }
}
class SalesOrder
{
public void LoadItems()
{
List<SalesItem> SalesItems = new List<SalesItem>();
SalesItem salesitem = new SalesItem()
{
Name = "Ball",
Price = 12,
Quantity = 1
};
SalesItems.Add(salesitem);
salesitem = new SalesItem()
{
Name = "Ball",
Price = 36,
Quantity = 3
};
SalesItems.Add(salesitem);
salesitem = new SalesItem()
{
Name = "Bat",
Price = 50,
Quantity = 1
};
SalesItems.Add(salesitem);
salesitem = new SalesItem()
{
Name = "Ball",
Price = 84,
Quantity = 7
};
SalesItems.Add(salesitem);
salesitem = new SalesItem()
{
Name = "Bat",
Price = 150,
Quantity = 3
};
SalesItems.Add(salesitem);
GroupOrders(SalesItems);
}
public List<SalesItem> GroupOrders(List<SalesItem> SalesItems)
{
var list = from item in SalesItems
group item by item.Name into orders
select new SalesItem
{
Name = orders.Key,
Price = orders.Sum(X=>X.Price),
Quantity = orders.Sum(X=>X.Quantity)
};
List<SalesItem> resultList = new List<SalesItem>();
foreach (SalesItem saleitem in list)
{
resultList.Add(saleitem);
}
return resultList;
}
}

Mixing in new fields to an object without hard-coding its fields in a LINQ query?

I have a simple LINQ query here:
var Staffs = new[]
{
new { id = 1, name = "Jefferson", age = 42},
new { id = 2, name = "Jacobson", age = 54},
new { id = 3, name = "Zhang", age = 34}
};
var payroll = new[]
{
new { pid = 1, wage = 5000},
new { pid = 2, wage = 6500},
new { pid = 3, wage = 6700}
};
var q = from stf in Staffs
from pay in payroll
where stf.id == pay.pid
select new
{
stfObj = stf,
pay.pid,
pay.wage
};
Here, stfObj would be an object containing the id, name and age fields
Here comes the question:
Is it possible to turn the object into the fields themselves without explicitly hard-coding the field names like this:
select new
{
stf.id,
stf.name,
stf.age,
pay.pid,
pay.wage
};
In this way, there will be no need to change the select new block when I add a new field to Staffs, like Gender for example
Is that possible?
(ok, this looks like the question here... anyway, hoping to get better answers here)

Is this what you want!
select new
{
sId=stfObj.id,
sName=stfObj.name,
sAge=stdObj.age,
pId=pay.pid,
pWage=pay.wage
};

Why not simply embed your object ?
select new {
staff = stf,
pay = pay
};

I do not know, what you need this for. But you could try to use Dictionary<> for this. Say, we have a class:
public class Test
{
public string Name { get; set; }
public string Desc { get; set; }
}
So you can do the following:
List<Test> list = new List<Test>
{
new Test
{
Name = "Test 1",
Desc = "Desc 1"
}
};
var temp = list.Select(t =>
{
Dictionary<string, object> values = new Dictionary<string, object>();
foreach (PropertyInfo pi in t.GetType().GetProperties())
values[pi.Name] = pi.GetValue(t, null);
return values;
})
.FirstOrDefault();
temp.ToList().ForEach(p => Console.WriteLine(string.Format("{0}:\t{1}", p.Key, p.Value)));
So if you add a property to the Test class, say, like this:
public bool Extra { get; set; }
You'll get it in the dictionary automatically. Probably you'll have to work with reflection methods overloads to get exactly what you need...