我无法找到这个错误的根源,因为当附加调试器时,它似乎没有发生。
修改集合;枚举操作可能无法执行
下面是代码。
这是Windows服务中的WCF服务器。只要有数据事件,服务就会调用NotifySubscribers()方法(随机间隔,但不经常——大约每天800次)。
When a Windows Forms client subscribes, the subscriber ID is added to the subscribers dictionary, and when the client unsubscribes, it is deleted from the dictionary. The error happens when (or after) a client unsubscribes. It appears that the next time the NotifySubscribers() method is called, the foreach() loop fails with the error in the subject line. The method writes the error into the application log as shown in the code below. When a debugger is attached and a client unsubscribes, the code executes fine.
您认为这段代码有问题吗?我需要使字典线程安全吗?
[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class SubscriptionServer : ISubscriptionServer
{
private static IDictionary<Guid, Subscriber> subscribers;
public SubscriptionServer()
{
subscribers = new Dictionary<Guid, Subscriber>();
}
public void NotifySubscribers(DataRecord sr)
{
foreach(Subscriber s in subscribers.Values)
{
try
{
s.Callback.SignalData(sr);
}
catch (Exception e)
{
DCS.WriteToApplicationLog(e.Message,
System.Diagnostics.EventLogEntryType.Error);
UnsubscribeEvent(s.ClientId);
}
}
}
public Guid SubscribeEvent(string clientDescription)
{
Subscriber subscriber = new Subscriber();
subscriber.Callback = OperationContext.Current.
GetCallbackChannel<IDCSCallback>();
subscribers.Add(subscriber.ClientId, subscriber);
return subscriber.ClientId;
}
public void UnsubscribeEvent(Guid clientId)
{
try
{
subscribers.Remove(clientId);
}
catch(Exception e)
{
System.Diagnostics.Debug.WriteLine("Unsubscribe Error " +
e.Message);
}
}
}
下面是一个特定的场景,需要使用专门的方法:
字典经常被列举出来。
字典很少被修改。
In this scenario creating a copy of the Dictionary (or the Dictionary.Values) before every enumeration can be quite costly. My idea about solving this problem is to reuse the same cached copy in multiple enumerations, and watch an IEnumerator of the original Dictionary for exceptions. The enumerator will be cached along with the copied data, and interrogated before starting a new enumeration. In case of an exception the cached copy will be discarded, and a new one will be created. Here is my implementation of this idea:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
public class EnumerableSnapshot<T> : IEnumerable<T>, IDisposable
{
private IEnumerable<T> _source;
private IEnumerator<T> _enumerator;
private ReadOnlyCollection<T> _cached;
public EnumerableSnapshot(IEnumerable<T> source)
{
_source = source ?? throw new ArgumentNullException(nameof(source));
}
public IEnumerator<T> GetEnumerator()
{
if (_source == null) throw new ObjectDisposedException(this.GetType().Name);
if (_enumerator == null)
{
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
else
{
var modified = false;
if (_source is ICollection collection) // C# 7 syntax
{
modified = _cached.Count != collection.Count;
}
if (!modified)
{
try
{
_enumerator.MoveNext();
}
catch (InvalidOperationException)
{
modified = true;
}
}
if (modified)
{
_enumerator.Dispose();
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
}
return _cached.GetEnumerator();
}
public void Dispose()
{
_enumerator?.Dispose();
_enumerator = null;
_cached = null;
_source = null;
}
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}
public static class EnumerableSnapshotExtensions
{
public static EnumerableSnapshot<T> ToEnumerableSnapshot<T>(
this IEnumerable<T> source) => new EnumerableSnapshot<T>(source);
}
使用的例子:
private static IDictionary<Guid, Subscriber> _subscribers;
private static EnumerableSnapshot<Subscriber> _subscribersSnapshot;
//...(in the constructor)
_subscribers = new Dictionary<Guid, Subscriber>();
_subscribersSnapshot = _subscribers.Values.ToEnumerableSnapshot();
// ...(elsewere)
foreach (var subscriber in _subscribersSnapshot)
{
//...
}
Unfortunately this idea cannot be used currently with the class Dictionary in .NET Core 3.0, because this class does not throw a Collection was modified exception when enumerated and the methods Remove and Clear are invoked. All other containers I checked are behaving consistently. I checked systematically these classes:
List<T>, Collection<T>, ObservableCollection<T>, HashSet<T>, SortedSet<T>, Dictionary<T,V> and SortedDictionary<T,V>. Only the two aforementioned methods of the Dictionary class in .NET Core are not invalidating the enumeration.
更新:通过比较缓存和原始集合的长度,我修复了上述问题。这个修正假设字典将直接作为参数传递给EnumerableSnapshot的构造函数,并且它的标识不会被(例如)像:dictionary这样的投影隐藏。选择(e => e).ΤοEnumerableSnapshot()。
重要:上面的类不是线程安全的。它旨在从专门运行在单个线程中的代码中使用。
公认的答案在最坏的情况下是不精确和不正确的。如果在ToList()期间进行了更改,您仍然会得到一个错误。除了锁(如果您有一个公共成员,则需要考虑锁的性能和线程安全性)之外,一个合适的解决方案是使用不可变类型。
一般来说,不可变类型意味着一旦创建了它,就不能更改它的状态。
所以你的代码应该是这样的:
public class SubscriptionServer : ISubscriptionServer
{
private static ImmutableDictionary<Guid, Subscriber> subscribers = ImmutableDictionary<Guid, Subscriber>.Empty;
public void SubscribeEvent(string id)
{
subscribers = subscribers.Add(Guid.NewGuid(), new Subscriber());
}
public void NotifyEvent()
{
foreach(var sub in subscribers.Values)
{
//.....This is always safe
}
}
//.........
}
如果您有一个公共成员,这可能特别有用。其他类总是可以对不可变类型进行foreach,而不用担心集合被修改。