关于list 和 IEnumerable 在lambd表达式的使用与注意

例子1：
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Linq.Expressions;
using System.Windows;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    /// 
    /// 
    public partial class MainWindow : Window
    {

        List<Person> persons=new List<Person>(); 
        public MainWindow()
        {
            InitializeComponent();
            persons.Add(new Person() {Age = 1, Name = "A1"});
            persons.Add(new Person() { Age = 2, Name = "A2" });
            persons.Add(new Person() { Age = 3, Name = "A3" });
            persons.Add(new Person() { Age = 4, Name = "A4" });
            persons.Add(new Person() { Age = 5, Name = "A5" });
            persons.Add(new Person() { Age = 5, Name = "A6" });
            persons.Add(new Person() { Age = 5, Name = "A7" });
            persons.Add(new Person() { Age = 6, Name = "A8" });
            Loaded += Onload;
        }
     
        private void Onload(object sender, RoutedEventArgs e)
        {
            var lists = persons.Select(p => p.Age > 2 ? p : null).Where(p=>p!=null);

            foreach (var list in lists)
            {
                Debug.WriteLine(list.Name);
            }
        }
    }

    public class Person
    {
        public int Age { get; set; }
        public string Name { get; set; }
    }

}

例子2：
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Linq.Expressions;
using System.Windows;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    /// 
    /// 
    public partial class MainWindow : Window
    {

        List<Person> persons=new List<Person>(); 
        public MainWindow()
        {
            InitializeComponent();
            persons.Add(new Person() {Age = 1, Name = "A1"});
            persons.Add(new Person() { Age = 2, Name = "A2" });
            persons.Add(new Person() { Age = 3, Name = "A3" });
            persons.Add(new Person() { Age = 4, Name = "A4" });
            persons.Add(new Person() { Age = 5, Name = "A5" });
            persons.Add(new Person() { Age = 5, Name = "A6" });
            persons.Add(new Person() { Age = 5, Name = "A7" });
            persons.Add(new Person() { Age = 6, Name = "A8" });
            Loaded += Onload;
        }
     
        private void Onload(object sender, RoutedEventArgs e)
        {
            var lists = persons.Select(p => p.Age > 2 ? p : null).Where(p=>p!=null).ToList();

            foreach (var list in lists)
            {
                Debug.WriteLine(list.Name);
            }
        }
    }

    public class Person
    {
        public int Age { get; set; }
        public string Name { get; set; }
    }

}

例子1和例子2区别在与lambda表达式后的.tolist();

重点来了

例子1中没有使用tolist所以当var lists = persons.Select(p => p.Age > 2 ? p : null).Where(p=>p!=null);

lists是没有值的只有当进行foreach循环时才有值，而例子2，lists是一个list，这样就会有内存分配给lists而例子1则不会，

总结当tolist有万条以上时要有例子1的写法，这样就会节省内存，但如果.select.......这个lambda表达式算法超复杂，那最后有例子2直接操作list来处理这样cpu的使用会减少很多。