I've been searching the difference between Select and SelectMany but I haven't been able to find a suitable answer. I need to learn the difference when using LINQ To SQL but all I've found are standard array examples.
Can someone provide a LINQ To SQL example?
SelectMany flattens queries that return lists of lists. For example
public class PhoneNumber
{
public string Number { get; set; }
}
public class Person
{
public IEnumerable<PhoneNumber> PhoneNumbers { get; set; }
public string Name { get; set; }
}
IEnumerable<Person> people = new List<Person>();
// Select gets a list of lists of phone numbers
IEnumerable<IEnumerable<PhoneNumber>> phoneLists = people.Select(p => p.PhoneNumbers);
// SelectMany flattens it to just a list of phone numbers.
IEnumerable<PhoneNumber> phoneNumbers = people.SelectMany(p => p.PhoneNumbers);
// And to include data from the parent in the result:
// pass an expression to the second parameter (resultSelector) in the overload:
var directory = people
.SelectMany(p => p.PhoneNumbers,
(parent, child) => new { parent.Name, child.Number });
Select many is like cross join operation in SQL where it takes the cross product.
For example if we have
Set A={a,b,c}
Set B={x,y}
Select many can be used to get the following set
{ (x,a) , (x,b) , (x,c) , (y,a) , (y,b) , (y,c) }
Note that here we take the all the possible combinations that can be made from the elements of set A and set B.
Here is a LINQ example you can try
List<string> animals = new List<string>() { "cat", "dog", "donkey" };
List<int> number = new List<int>() { 10, 20 };
var mix = number.SelectMany(num => animals, (n, a) => new { n, a });
the mix will have following elements in flat structure like
{(10,cat), (10,dog), (10,donkey), (20,cat), (20,dog), (20,donkey)}
SelectMany() lets you collapse a multidimensional sequence in a way that would otherwise require a second Select() or loop.
More details at this blog post.
There are several overloads to SelectMany. One of them allows you to keep trace of any relationship between parent and children while traversing the hierarchy.
Example: suppose you have the following structure: League -> Teams -> Player.
You can easily return a flat collection of players. However you may lose any reference to the team the player is part of.
Fortunately there is an overload for such purpose:
var teamsAndTheirLeagues =
from helper in leagues.SelectMany
( l => l.Teams
, ( league, team ) => new { league, team } )
where helper.team.Players.Count > 2
&& helper.league.Teams.Count < 10
select new
{ LeagueID = helper.league.ID
, Team = helper.team
};
The previous example is taken from Dan's IK blog. I strongly recommend you take a look at it.
The SelectMany() method is used to flatten a sequence in which each of the elements of the sequence is a separate.
I have class user same like this
class User
{
public string UserName { get; set; }
public List<string> Roles { get; set; }
}
main:
var users = new List<User>
{
new User { UserName = "Reza" , Roles = new List<string>{"Superadmin" } },
new User { UserName = "Amin" , Roles = new List<string>{"Guest","Reseption" } },
new User { UserName = "Nima" , Roles = new List<string>{"Nurse","Guest" } },
};
var query = users.SelectMany(user => user.Roles, (user, role) => new { user.UserName, role });
foreach (var obj in query)
{
Console.WriteLine(obj);
}
//output
//{ UserName = Reza, role = Superadmin }
//{ UserName = Amin, role = Guest }
//{ UserName = Amin, role = Reseption }
//{ UserName = Nima, role = Nurse }
//{ UserName = Nima, role = Guest }
You can use operations on any item of sequence
int[][] numbers = {
new[] {1, 2, 3},
new[] {4},
new[] {5, 6 , 6 , 2 , 7, 8},
new[] {12, 14}
};
IEnumerable<int> result = numbers
.SelectMany(array => array.Distinct())
.OrderBy(x => x);
//output
//{ 1, 2 , 2 , 3, 4, 5, 6, 7, 8, 12, 14 }
List<List<int>> numbers = new List<List<int>> {
new List<int> {1, 2, 3},
new List<int> {12},
new List<int> {5, 6, 5, 7},
new List<int> {10, 10, 10, 12}
};
IEnumerable<int> result = numbers
.SelectMany(list => list)
.Distinct()
.OrderBy(x=>x);
//output
// { 1, 2, 3, 5, 6, 7, 10, 12 }
Some SelectMany may not be necessary. Below 2 queries give the same result.
Customers.Where(c=>c.Name=="Tom").SelectMany(c=>c.Orders)
Orders.Where(o=>o.Customer.Name=="Tom")
For 1-to-Many relationship,
from o in Orders
join c in Customers on o.CustomerID equals c.ID
where c.Name == "Tom"
select o
Without getting too technical - database with many Organizations, each with many Users:-
var orgId = "123456789";
var userList1 = db.Organizations
.Where(a => a.OrganizationId == orgId)
.SelectMany(a => a.Users)
.ToList();
var userList2 = db.Users
.Where(a => a.OrganizationId == orgId)
.ToList();
both return the same ApplicationUser list for the selected Organization.
The first "projects" from Organization to Users, the second queries the Users table directly.
Consider this example :
var array = new string[2]
{
"I like what I like",
"I like what you like"
};
//query1 returns two elements sth like this:
//fisrt element would be array[5] :[0] = "I" "like" "what" "I" "like"
//second element would be array[5] :[1] = "I" "like" "what" "you" "like"
IEnumerable<string[]> query1 = array.Select(s => s.Split(' ')).Distinct();
//query2 return back flat result sth like this :
// "I" "like" "what" "you"
IEnumerable<string> query2 = array.SelectMany(s => s.Split(' ')).Distinct();
So as you see duplicate values like "I" or "like" have been removed from query2 because "SelectMany" flattens and projects across multiple sequences. But query1 returns sequence of string arrays. and since there are two different arrays in query1 (first and second element), nothing would be removed.
One more example how SelectMany + Select can be used in order to accumulate sub array objects data.
Suppose we have users with they phones:
class Phone {
public string BasePart = "555-xxx-xxx";
}
class User {
public string Name = "Xxxxx";
public List<Phone> Phones;
}
Now we need to select all phones' BaseParts of all users:
var usersArray = new List<User>(); // array of arrays
List<string> allBaseParts = usersArray.SelectMany(ua => ua.Phones).Select(p => p.BasePart).ToList();
The SelectMany method knocks down an IEnumerable<IEnumerable<T>> into an IEnumerable<T>, like communism, every element is behaved in the same manner(a stupid guy has same rights of a genious one).
var words = new [] { "a,b,c", "d,e", "f" };
var splitAndCombine = words.SelectMany(x => x.Split(','));
// returns { "a", "b", "c", "d", "e", "f" }
Here is a code example with an initialized small collection for testing:
class Program
{
static void Main(string[] args)
{
List<Order> orders = new List<Order>
{
new Order
{
OrderID = "orderID1",
OrderLines = new List<OrderLine>
{
new OrderLine
{
ProductSKU = "SKU1",
Quantity = 1
},
new OrderLine
{
ProductSKU = "SKU2",
Quantity = 2
},
new OrderLine
{
ProductSKU = "SKU3",
Quantity = 3
}
}
},
new Order
{
OrderID = "orderID2",
OrderLines = new List<OrderLine>
{
new OrderLine
{
ProductSKU = "SKU4",
Quantity = 4
},
new OrderLine
{
ProductSKU = "SKU5",
Quantity = 5
}
}
}
};
//required result is the list of all SKUs in orders
List<string> allSKUs = new List<string>();
//With Select case 2 foreach loops are required
var flattenedOrdersLinesSelectCase = orders.Select(o => o.OrderLines);
foreach (var flattenedOrderLine in flattenedOrdersLinesSelectCase)
{
foreach (OrderLine orderLine in flattenedOrderLine)
{
allSKUs.Add(orderLine.ProductSKU);
}
}
//With SelectMany case only one foreach loop is required
allSKUs = new List<string>();
var flattenedOrdersLinesSelectManyCase = orders.SelectMany(o => o.OrderLines);
foreach (var flattenedOrderLine in flattenedOrdersLinesSelectManyCase)
{
allSKUs.Add(flattenedOrderLine.ProductSKU);
}
//If the required result is flattened list which has OrderID, ProductSKU and Quantity,
//SelectMany with selector is very helpful to get the required result
//and allows avoiding own For loops what according to my experience do code faster when
// hundreds of thousands of data rows must be operated
List<OrderLineForReport> ordersLinesForReport = (List<OrderLineForReport>)orders.SelectMany(o => o.OrderLines,
(o, ol) => new OrderLineForReport
{
OrderID = o.OrderID,
ProductSKU = ol.ProductSKU,
Quantity = ol.Quantity
}).ToList();
}
}
class Order
{
public string OrderID { get; set; }
public List<OrderLine> OrderLines { get; set; }
}
class OrderLine
{
public string ProductSKU { get; set; }
public int Quantity { get; set; }
}
class OrderLineForReport
{
public string OrderID { get; set; }
public string ProductSKU { get; set; }
public int Quantity { get; set; }
}
Yet another way to look at it.
var src = new[] { 1, 2, 3, };
var map = src.Select(i => i.ToString());
var flatMap = src.SelectMany(i => /* go wild here producing IEnumerable's */);
var empty = src.SelectMany(_ => new string[0]);
The length of map will always match the length of src. An element of map will be whatever the transformation is - string in the above example, and if it's some IEnumerable it'll still be "wrapped".
The length of flatMap on the other hand will be whatever you want it to be according to the transformation provided for each element as all those iterables will be "squashed" into one. And so in the extreme the length of empty will be 0 despite the fact that src.Length = 3.
So SelectMany can be thought of as discarding the original IEnumerable.
