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Simple C# console showing what happens when using LINQ Select
using System;
using System.Collections.Generic;
using System.Linq;
namespace LinqProjectionsTest
{
class Program
{
static void Main(string[] args)
{
var list = new List<int> {
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
};
Console.WriteLine("list.Select(...) starting");
var projected = list.Select(num =>
{
var projection = num + 20;
Console.WriteLine("{0} was selected", projection);
return new
{
Original = num,
Projection = projection
};
}).Select(num =>
{
var projection = num.Projection * 2;
Console.WriteLine("{0} was selected", projection);
return new
{
Original = num,
Projection = projection
};
});
Console.WriteLine("list.Select(...) ending");
// You will notice that you will see the "list.Select(...) starting" immediately followed by
// the "list.Select(...) ending" in your output.
//
// This is because the expressions in Select statements are lazy and only evaluated as they
// are enumerated.
//
// You should expect to see the logged lines when we enumerate them as per
// the below lines.
foreach (var item in projected)
{
Console.WriteLine("{0}, {1} was enumerated", item.Original, item.Projection);
}
// When we call ToList() we are forcing the framework to enumerate our list and put the
// results in memory.
// We will see the logging from each select statement between the "projected.ToList() start"
// and "projected.ToList() end" log statements
Console.WriteLine("projected.ToList() start");
var projectedList = projected.ToList();
Console.WriteLine("projected.ToList() finished");
// When we enumerate our in-memory list we will not see the intermediate logged statements in our
// select blocks. This is because we have forced evaluation with ToList(), in this example code
// we have simply moved the evaluation of our Select to before we enumerate but also forced that
// memory is allocated for us. This is unneccesary if we only wanted to enumerate the collection
// once
foreach (var item in projectedList)
{
Console.WriteLine("{0}, {1} was enumerated", item.Original, item.Projection);
}
Console.ReadLine();
}
}
}
list.Select(...) starting
list.Select(...) ending
20 was selected
40 was selected
{ Original = 0, Projection = 20 }, 40 was enumerated
21 was selected
42 was selected
{ Original = 1, Projection = 21 }, 42 was enumerated
22 was selected
44 was selected
{ Original = 2, Projection = 22 }, 44 was enumerated
23 was selected
46 was selected
{ Original = 3, Projection = 23 }, 46 was enumerated
24 was selected
48 was selected
{ Original = 4, Projection = 24 }, 48 was enumerated
25 was selected
50 was selected
{ Original = 5, Projection = 25 }, 50 was enumerated
26 was selected
52 was selected
{ Original = 6, Projection = 26 }, 52 was enumerated
27 was selected
54 was selected
{ Original = 7, Projection = 27 }, 54 was enumerated
28 was selected
56 was selected
{ Original = 8, Projection = 28 }, 56 was enumerated
29 was selected
58 was selected
{ Original = 9, Projection = 29 }, 58 was enumerated
30 was selected
60 was selected
{ Original = 10, Projection = 30 }, 60 was enumerated
projected.ToList() start
20 was selected
40 was selected
21 was selected
42 was selected
22 was selected
44 was selected
23 was selected
46 was selected
24 was selected
48 was selected
25 was selected
50 was selected
26 was selected
52 was selected
27 was selected
54 was selected
28 was selected
56 was selected
29 was selected
58 was selected
30 was selected
60 was selected
projected.ToList() finished
{ Original = 0, Projection = 20 }, 40 was enumerated
{ Original = 1, Projection = 21 }, 42 was enumerated
{ Original = 2, Projection = 22 }, 44 was enumerated
{ Original = 3, Projection = 23 }, 46 was enumerated
{ Original = 4, Projection = 24 }, 48 was enumerated
{ Original = 5, Projection = 25 }, 50 was enumerated
{ Original = 6, Projection = 26 }, 52 was enumerated
{ Original = 7, Projection = 27 }, 54 was enumerated
{ Original = 8, Projection = 28 }, 56 was enumerated
{ Original = 9, Projection = 29 }, 58 was enumerated
{ Original = 10, Projection = 30 }, 60 was enumerated
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