一个Linq 的 ToList()问题

Question

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            List<CategoryLevel> allLevels = new List<CategoryLevel>();
            allLevels.Add(new CategoryLevel
            {
                CategoryLevelId = 1,
                CategoryId = 1,
                LevelName = "Level1"
            });
            allLevels.Add(new CategoryLevel
            {
                CategoryLevelId = 2,
                CategoryId = 2,
                LevelName = "Level2"
            });
            List<Category> allCategories = new List<Category>();
            allCategories.Add(new Category
            {
                CategoryId = 1
            });
            allCategories.Add(new Category
            {
                CategoryId = 2
            });
            foreach (var category in allCategories)
            {
                category.CategoryLevels = allLevels.Where(l => l.CategoryId == category.CategoryId); //为什么如果这里加ToList() 后就不会失败
            }
            var checkedLevels = allCategories.Where(c => c.CategoryId == 1).First().CategoryLevels;
            Debug.Assert(checkedLevels.Where(l => l.CategoryLevelId == 2).Count() == 0); //断言失败, 为什么???
        }
    }
    internal class Category
    {
        public int CategoryId { get; set; }
        public IEnumerable<CategoryLevel> CategoryLevels { get; set; }
    }

    internal class CategoryLevel
    {
        public int CategoryLevelId { get; set; }
        public int CategoryId { get; set; }
        public string LevelName { get; set; }
    }
}

Answer 1

因为 checkedLevels.Where(l => l.CategoryLevelId == 2).Count() 的结果是"1"

断言1 == 0 当然是失败的

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起云

Answer 2

我知道 checkedLevels.Where(l => l.CategoryLevelId == 2).Count()的结果是"1", 我想知道为什么加了ToList以后得到的结果是0, 而不加得到的结果是1.

Answer 3

这是一个延迟执行（Deferred excution）的问题。当调用where时，实际得到的不是集合，而是运算表达式，直到ToList时，他才成为对应的集合。在你的循环中，运算式中的变量值发生了变化(category)，等同于如下代码，所以 levellist1 levellist2在执行时运算了相同的结果。

            var cate1 = allCategories[0];
            var level1 = allLevels.Where(l => l.CategoryId == cate1.CategoryId);

            cate1 = allCategories[1];
            var level2 = allLevels.Where(l => l.CategoryId == cate1.CategoryId);

            var levellist1 = level1.ToList();
            var levellist2 = level2.ToList();

你改成这样，结果就对了

            var cate1 = allCategories[0];
            var level1 = allLevels.Where(l => l.CategoryId == cate1.CategoryId);

            var levellist1 = level1.ToList();

            cate1 = allCategories[1];
            var level2 = allLevels.Where(l => l.CategoryId == cate1.CategoryId);

            var levellist2 = level2.ToList();

其实，也就对应于你把 ToList() 放到循环里。所以，在使用deferred excution时，小心中间变量变化的副作用。

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Mog Liang