using System;
using System.Collections.Generic;
using System.Linq;
public class MainClass {
public static void Main() {
var numbers = new List<int>();
numbers.Add(1);
IEnumerable<int> query = numbers.Select(n => n * 10);
numbers.Add(2);
foreach (int n in query)
Console.Write(n + "|");
}
}
LINQ
A deferred execution query is reevaluated when you reenumerate:
using System;
using System.Collections.Generic;
using System.Linq;
public class MainClass {
public static void Main() {
var numbers = new List<int>() { 1, 2 };
IEnumerable<int> query = numbers.Select(n => n * 10);
foreach (int n in query) Console.Write(n + "|");
numbers.Clear();
foreach (int n in query) Console.Write(n + "|");
}
}
Immediate Query Execution
using System;
using System.Linq;
class MainClass {
static double Square(double n) {
Console.WriteLine("Computing Square(" + n + ")...");
return Math.Pow(n, 2);
}
public static void Main() {
int[] numbers = { 1, 2, 3 };
var query = from n in numbers select Square(n);
foreach (var n in query.ToList())
Console.WriteLine(n);
}
}
Deferred Query Execution
using System;
using System.Linq;
class MainClass {
static double Square(double n) {
return Math.Pow(n, 2);
}
public static void Main() {
int[] numbers = { 1, 2, 3 };
var query = from n in numbers select Square(n);
foreach (var n in query)
Console.WriteLine(n);
}
}
Shows how queries can be reused
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
public class MainClass {
public static void Main() {
int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
var lowNumbers =
from n in numbers
where n <= 3
select n;
Console.WriteLine("First run numbers <= 3:");
foreach (int n in lowNumbers) {
Console.WriteLine(n);
}
for (int i = 0; i < 10; i++) {
numbers[i] = -numbers[i];
}
Console.WriteLine("Second run numbers <= 3:");
foreach (int n in lowNumbers) {
Console.WriteLine(n);
}
}
}
[/csharp]
How queries can be executed immediately with operators such as ToList().
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
public class MainClass {
public static void Main() {
int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
int i = 0;
var q = (
from n in numbers
select ++i)
.ToList();
// The local variable i has already been fully
// incremented before we iterate the results:
foreach (var v in q) {
Console.WriteLine("v = {0}, i = {1}", v, i);
}
}
}
Deferred Query Execution: shows how query execution is deferred until the query is enumerated at a foreach statement.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
public class MainClass {
public static void Main() {
int[] numbers = new int[] { 5, 4, 1, 3, 9 };
int i = 0;
var q =
from n in numbers
select ++i;
foreach (var v in q) {
Console.WriteLine("v = {0}, i = {1}", v, i);
}
}
}