我们知道，有时候在一些项目中，为了性能，往往会一次性加载很多条记录来进行循环处理（备注：而非列表呈现）。比如：从数据库中加载 10000 个用户，并且每个用户包含了 20 个“爱好”，在 WinForm 界面我们需要用进度条的方式实时显示每个用户处理的进度，这时候当然是一次性加载很多条记录来进行循环处理更加快速。
通常，我们会先 foreach 从数据库中加载的用户集合，然后从“爱好”集合中筛选出某个用户的所有爱好来进行处理。比如如下代码：
foreach (var userItem in userList) { List<Hobby> tempHobbyList = hobbyList.Where(c => c.UserId == userItem.Id).ToList(); foreach (var hobbyItem in tempHobbyList) { string hobbyName = hobbyItem.HobbyName; //...其它的代码 } }
如下图，我们编写了如上面的代码来进行实际测试，结果发现，当 userList 足够多、或者 hobbyList 足够多，或者 userList、hobbyList 同时都足够多时，性能会大大的降低，这时候就需要我们改变方式，采用先把 hobbyList 安装 userId 进行分组（GroupBy）变成 Dictionary<int, List<Hobby>>，然后在 userList 的循环体中直接从 Dictionary<int, List<Hobby>> 从检索，这样性能会大大提高。
namespace ConsAppTest { class User { public int Id { get; set; } public string Name { get; set; } public int? Age { get; set; } } class Hobby { public int HobbyId { get; set; } public string HobbyName { get; set; } public int UserId { get; set; } } class Program { private const int UserCountForNew = 6000; private const int HobbyCountForEveryUser = 20; private static async Task Main(string[] args) { string messageTemplate = $"产生 { UserCountForNew } 个虚拟用户，并且每个虚拟用户产生了 { HobbyCountForEveryUser } 个“爱好”共耗时 {{0}} 秒。"; //下面是方案1 - 开始 Stopwatch globalWatch1 = Stopwatch.StartNew(); List<User> userList1; List<Hobby> hobbyList1; CreateUserListAndHobbyList(out userList1, out hobbyList1); globalWatch1.Stop(); double totalSeconds1 = (double)globalWatch1.ElapsedMilliseconds / 1000; Console.WriteLine(messageTemplate, totalSeconds1.ToString("#.##")); Stopwatch stage1Watch = Stopwatch.StartNew(); foreach (var userItem in userList1) { List<Hobby> tempHobbyList = hobbyList1.Where(c => c.UserId == userItem.Id).ToList(); foreach (var hobbyItem in tempHobbyList) { string hobbyName = hobbyItem.HobbyName; } } stage1Watch.Stop(); double stage1WatchTotalSeconds = (double)stage1Watch.ElapsedMilliseconds / 1000; Console.WriteLine("方案1总耗时：{0}\n\n", stage1WatchTotalSeconds.ToString("#.##")); //下面是方案1 - 结束 //下面是方案2 - 开始 Stopwatch globalWatch2 = Stopwatch.StartNew(); List<User> userList2; List<Hobby> hobbyList2; CreateUserListAndHobbyList(out userList2, out hobbyList2); globalWatch2.Stop(); double totalSeconds2 = (double)globalWatch2.ElapsedMilliseconds / 1000; Console.WriteLine(messageTemplate, totalSeconds2.ToString("#.##")); Stopwatch stage2Watch = Stopwatch.StartNew(); foreach (var userItem in userList2) { List<Hobby> tempHobbyList = hobbyList2.Where(c => c.UserId == userItem.Id).ToList(); foreach (var hobbyItem in tempHobbyList) { string hobbyName = hobbyItem.HobbyName; } hobbyList2.RemoveAll(c => c.UserId == userItem.Id); } stage2Watch.Stop(); double stage2WatchTotalSeconds = (double)stage2Watch.ElapsedMilliseconds / 1000; Console.WriteLine("方案2总耗时：{0}\n\n", stage2WatchTotalSeconds.ToString("#.##")); //下面是方案2 - 结束 //下面是方案3 - 开始 Stopwatch globalWatch3 = Stopwatch.StartNew(); List<User> userList3; List<Hobby> hobbyList3; CreateUserListAndHobbyList(out userList3, out hobbyList3); globalWatch3.Stop(); double totalSeconds3 = (double)globalWatch3.ElapsedMilliseconds / 1000; Console.WriteLine(messageTemplate, totalSeconds3.ToString("#.##")); Stopwatch stage3Watch = Stopwatch.StartNew(); Dictionary<int, List<Hobby>> keyValuePairs = hobbyList3.GroupBy<Hobby, int>(c => c.UserId).ToDictionary(c => c.Key, c => c.ToList()); foreach (var userItem in userList3) { List<Hobby> tempHobbyList = keyValuePairs[userItem.Id]; foreach (var hobbyItem in tempHobbyList) { string hobbyName = hobbyItem.HobbyName; } } stage3Watch.Stop(); double stage3WatchTotalSeconds = (double)stage3Watch.ElapsedMilliseconds / 1000; Console.WriteLine("方案3总耗时：{0}\n\n", stage3WatchTotalSeconds.ToString("#.##")); //下面是方案3 - 结束 Console.ReadLine(); } private static void CreateUserListAndHobbyList(out List<User> rightUserList, out List<Hobby> rightHobbyList) { List<User> userList = new List<User>(); List<Hobby> hobbyList = new List<Hobby>(); int hobbyIndex = 0; for (int i = 1; i <= UserCountForNew; i++) { int userId = i; userList.Add(new User { Id = userId, Name = "张三" + userId, Age = userId }); for (int j = 0; j < HobbyCountForEveryUser; j++) { hobbyIndex++; hobbyList.Add(new Hobby { HobbyId = hobbyIndex, HobbyName = "唱歌" + hobbyIndex, UserId = userId }); } } rightUserList = userList; rightHobbyList = hobbyList; } } }
最终，我们看到，这三种方案，每种方案都产生 6000 个虚拟用户，并且每个虚拟用户产生了 20 个“爱好”。第一二种方案检索的速度要 8秒、9秒左右，而第三种方案从 Dictionary<int, List<Hobby>> 中检索只需要 0.01 到 0.02 秒，可谓是性能大大提升。
谢谢浏览！
实例演示 C# 中 Dictionary<Key, Value> 的检索速度远远大于 hobbyList.Where(c => c.UserId == user.Id)
原文：https://www.cnblogs.com/Music/p/fast-dictionary-for-searching-in-csharp.html