Linq101-QueryExecution

using System;
using System.Linq;

namespace Linq101
{
    class QueryExecution
    {
        /// <summary>
        /// The following sample shows how query execution is deferred until the query is enumerated at a foreach statement.
        /// </summary>
        public void Linq99()
        {
            int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

            int i = 0;

            //延迟执行
            var q = from n in numbers select ++i;

            foreach (int n in q)
            {
                Console.WriteLine("n={0},i={1}", n, i);
            }
        }

        /// <summary>
        /// The following sample shows how queries can be executed immediately with operators such as ToList().
        /// </summary>
        public void Linq100()
        {
            int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

            int i = 0;

            //ToList->立即执行
            var q = (from n in numbers select ++i).ToList();

            foreach (int n in q)
            {
                Console.WriteLine("n={0},i={1}", n, i);
            }
        }

        /// <summary>
        /// The following sample shows how, because of deferred execution, queries can be used again after data changes and will then operate on the new data.
        /// </summary>
        public void Linq101()
        {
            int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

            //可重用
            var lowNumbers = from n in numbers
                             where n <= 3
                             select n;

            Console.WriteLine("First run numbers <=3:");
            foreach (int number in lowNumbers)
            {
                Console.WriteLine(number);
            }

            lowNumbers = (from n in numbers select -n).ToArray();

            Console.WriteLine("Second run numbers <=3:");
            foreach (int number in lowNumbers)
            {
                Console.WriteLine(number);
            }
        }
    }
}