Select and select many difference
I have read many sites/threads on select and select many in LIN开发者_Python百科Q but still don't quite understand.
Does select return one element in a collection and select many flatten a collection (eg List>())?
Thanks
Here's a sample. Hope it clarifies everything:
static void MethodRun()
{
List<Topping> testToppings = new List<Topping> { Topping.Cheese, Topping.Pepperoni, Topping.Sausage };
var firstLetterofToppings = testToppings.Select(top => top.ToString().First());
// returns "C, P, S"
var singleToppingPizzas = testToppings.Select(top => new Pizza(top)).ToArray();
// returns "Pizza(Cheese), Pizza(Pepperoni), Pizza(Sausage)"
List<Topping> firstPizza = new List<Topping> { Topping.Cheese, Topping.Anchovies };
List<Topping> secondPizza = new List<Topping> { Topping.Sausage, Topping.CanadianBacon, Topping.Pepperoni };
List<Topping> thirdPizza = new List<Topping> { Topping.Ham, Topping.Pepperoni };
List<IEnumerable<Topping>> toppingsPurchaseOrder = new List<IEnumerable<Topping>> { firstPizza, secondPizza, thirdPizza };
var toppingsToOrder = toppingsPurchaseOrder.SelectMany(order => order);
//returns "Cheese, Anchovies, Sausage, CanadianBacon, Pepperoni, Ham, Pepperoni"
}
class Pizza
{
public List<Topping> Toppings { get; private set; }
public Pizza(Topping topping) : this(new List<Topping> { topping }) { }
public Pizza(IEnumerable<Topping> toppings)
{
this.Toppings = new List<Topping>();
this.Toppings.AddRange(toppings);
}
}
enum Topping
{
Cheese,
Pepperoni,
Anchovies,
Sausage,
Ham,
CanadianBacon
}
The key is that Select() can select any type of object. It's true that you can select a property of whatever generic value is assigned to your collection, but you can select any other type of object also. SelectMany() just flattens your list.
SelectMany returns a number of objects for each of the object passed into the expression. Select istead returns a single object for each object passed into the expression.
To quote the documentation:
Select Many
Projects each element of a sequence to an IEnumerable<(Of <(T>)>) and flattens the resulting sequences into one sequence.
Select
Projects each element of a sequence into a new form.
You can use SelectMany
if you want to flatten a hierarchy. E.g. if you have Orders
and OrderDetails
. If you want make a selection based upon the orders, but you want as a return the OrderDetails
use SelectMany
.
var result = db.Orders
.Where(x => x.CustomerId == 500) // input to next expression is IEnumerable<Order>
.SelectMany(x => x.OrderDetails) // input to next expression is IEnumerable<OrderDetails>
.Sum(x => x.PositionTotal);
var result = db.Orders
.Where(x => x.CustomerId == 500) // input to next expression is IEnumerable<Order>
.Select(x => CustomerName);
Let's go sequentially over your question:
1. Does select return one element in a collection? -> No, absolutely not. 'select' returns exactly the same number of elements that are in the collection, but into different shape(if wanted).
But, yes, it returns one sequence(/collection) having all those elements(e.g. even_sqare in below example).
e.g
int[] even = {2,4};
int[] even_square = even.Select(n=> n*2).ToArray();
o/p
even_square outputs to {4,8}
which is exact same in counts (2), but different projection, we gave squares of each by selecting them.
2. & does select many flatten a collection (e.g. List>())?
-> yes, but actually, it's like a cross join with our control.
int[] odd = { 1, 3 };
int[] even = { 2, 4 };
int[] crossjoin =
even.SelectMany(
n => odd, //combining even with odd
(n,o)=>Convert.ToInt32(n.ToString()+o.ToString())//given many selects, decide our projection
).ToArray();
foreach (var item in crossjoin)
{
Console.WriteLine(item);
}
Output: 21
23
41
43
Now a million dollar thing:
- Unlike most operators in linq, SelectMany takes two Collections, instead of one.
- "even" is first collection, and odd is the second collection, which we passed in using lambda like this 'n = > odd'. (this is where people say it flattens)
- third parameter(in above statement, second parameter) is TResult which gives us output which is cross join and that's beauty of SelectMany if we understood it.
- Enjoy learning.
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