我可以从该字典的枚举循环中删除 ConcurrentDictionary 中的项目吗? [英] Can I remove items from a ConcurrentDictionary from within an enumeration loop of that dictionary?
问题描述
例如:
ConcurrentDictionary<string,Payload> itemCache = GetItems();
foreach(KeyValuePair<string,Payload> kvPair in itemCache)
{
if(TestItemExpiry(kvPair.Value))
{ // Remove expired item.
itemCache.TryRemove(kvPair.Key, out Payload removedItem);
}
}
显然是用普通的 Dictionary
Obviously with an ordinary Dictionary<K,V> this will throw an exception, because removing items changes the dictionary's internal state during the life of the enumeration. It's my understanding that this is not the case for a ConcurrentDictionary, as the provided IEnumerable handles internal state changing. Am I understanding this right? Is there a better pattern to use?
推荐答案
我很奇怪您现在收到了两个似乎证实您不能这样做的答案.我只是自己测试过，它运行良好，没有抛出任何异常.
It's strange to me that you've now received two answers that seem to confirm you can't do this. I just tested it myself and it worked fine without throwing any exception.
下面是我用来测试行为的代码，然后是输出的摘录(大约在我按下C"以清除 foreach 和 S 之后立即停止后台线程).请注意，我对这个 ConcurrentDictionary 施加了相当大的压力:16 个线程计时器，每个线程大约每 15 毫秒尝试添加一个项目.
Below is the code I used to test the behavior, followed by an excerpt of the output (around when I pressed 'C' to clear the dictionary in a foreach and S immediately afterwards to stop the background threads). Notice that I put a pretty substantial amount of stress on this ConcurrentDictionary: 16 threading timers each attempting to add an item roughly every 15 milliseconds.
在我看来，这个类非常健壮，如果您在多线程场景中工作，则值得您关注.
It seems to me this class is quite robust, and worth your attention if you're working in a multithreaded scenario.
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
namespace ConcurrencySandbox {
class Program {
private const int NumConcurrentThreads = 16;
private const int TimerInterval = 15;
private static ConcurrentDictionary<int, int> _dictionary;
private static WaitHandle[] _timerReadyEvents;
private static Timer[] _timers;
private static volatile bool _timersRunning;
[ThreadStatic()]
private static Random _random;
private static Random GetRandom() {
return _random ?? (_random = new Random());
}
static Program() {
_dictionary = new ConcurrentDictionary<int, int>();
_timerReadyEvents = new WaitHandle[NumConcurrentThreads];
_timers = new Timer[NumConcurrentThreads];
for (int i = 0; i < _timerReadyEvents.Length; ++i)
_timerReadyEvents[i] = new ManualResetEvent(true);
for (int i = 0; i < _timers.Length; ++i)
_timers[i] = new Timer(RunTimer, _timerReadyEvents[i], Timeout.Infinite, Timeout.Infinite);
_timersRunning = false;
}
static void Main(string[] args) {
Console.Write("Press Enter to begin. Then press S to start/stop the timers, C to clear the dictionary, or Esc to quit.");
Console.ReadLine();
StartTimers();
ConsoleKey keyPressed;
do {
keyPressed = Console.ReadKey().Key;
switch (keyPressed) {
case ConsoleKey.S:
if (_timersRunning)
StopTimers(false);
else
StartTimers();
break;
case ConsoleKey.C:
Console.WriteLine("COUNT: {0}", _dictionary.Count);
foreach (var entry in _dictionary) {
int removedValue;
bool removed = _dictionary.TryRemove(entry.Key, out removedValue);
}
Console.WriteLine("COUNT: {0}", _dictionary.Count);
break;
}
} while (keyPressed != ConsoleKey.Escape);
StopTimers(true);
}
static void StartTimers() {
foreach (var timer in _timers)
timer.Change(0, TimerInterval);
_timersRunning = true;
}
static void StopTimers(bool waitForCompletion) {
foreach (var timer in _timers)
timer.Change(Timeout.Infinite, Timeout.Infinite);
if (waitForCompletion) {
WaitHandle.WaitAll(_timerReadyEvents);
}
_timersRunning = false;
}
static void RunTimer(object state) {
var readyEvent = state as ManualResetEvent;
if (readyEvent == null)
return;
try {
readyEvent.Reset();
var r = GetRandom();
var entry = new KeyValuePair<int, int>(r.Next(), r.Next());
if (_dictionary.TryAdd(entry.Key, entry.Value))
Console.WriteLine("Added entry: {0} - {1}", entry.Key, entry.Value);
else
Console.WriteLine("Unable to add entry: {0}", entry.Key);
} finally {
readyEvent.Set();
}
}
}
}
输出(摘录)
cAdded entry: 108011126 - 154069760 // <- pressed 'C'
Added entry: 245485808 - 1120608841
Added entry: 1285316085 - 656282422
Added entry: 1187997037 - 2096690006
Added entry: 1919684529 - 1012768429
Added entry: 1542690647 - 596573150
Added entry: 826218346 - 1115470462
Added entry: 1761075038 - 1913145460
Added entry: 457562817 - 669092760
COUNT: 2232 // <- foreach loop begins
COUNT: 0 // <- foreach loop ends
Added entry: 205679371 - 1891358222
Added entry: 32206560 - 306601210
Added entry: 1900476106 - 675997119
Added entry: 847548291 - 1875566386
Added entry: 808794556 - 1247784736
Added entry: 808272028 - 415012846
Added entry: 327837520 - 1373245916
Added entry: 1992836845 - 529422959
Added entry: 326453626 - 1243945958
Added entry: 1940746309 - 1892917475
另请注意，根据控制台输出，foreach 循环似乎锁定了其他试图向字典添加值的线程.(我可能是错的，否则我猜你会在COUNT"行之间看到一堆Added entry"行.)
Also note that, based on the console output, it looks like the foreach loop locked out the other threads that were trying to add values to the dictionary. (I could be wrong, but otherwise I would've guessed you would've seen a bunch of "Added entry" lines between the "COUNT" lines.)
这篇关于我可以从该字典的枚举循环中删除 ConcurrentDictionary 中的项目吗?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
