我編寫(xiě)了一些類(lèi)來(lái)使用SynchronizedCollection.class MultithreadTesting{ public readonly SynchronizedCollection<int> testlist = new SynchronizedCollection<int>(); public SynchronizedReadOnlyCollection<int> pubReadOnlyProperty { get { return new SynchronizedReadOnlyCollection<int>(testlist.SyncRoot, testlist); } } public void Test() { int numthreads = 20; Thread[] threads = new Thread[numthreads]; List<Task> taskList = new List<Task>(); for (int i = 0; i < numthreads / 2; i++) { taskList.Add(Task.Factory.StartNew(() => { for (int j = 0; j < 100000; j++) { testlist.Add(42); } })); } for (int i = numthreads / 2; i < numthreads; i++) { taskList.Add(Task.Factory.StartNew(() => { var sum = 0; foreach (int num in pubReadOnlyProperty) { sum += num; } })); } Task.WaitAll(taskList.ToArray()); testlist.Clear(); }}運(yùn)行它我使用 MultithreadTesting test = new MultithreadTesting(); while (true) test.Test();但是代碼讓我失望System.ArgumentException: 'Destination array was not long enough. Check destIndex and length, and the array's lower bounds.'如果我嘗試使用testlistin foreach，我會(huì)得到System.InvalidOperationException: 'Collection was modified; enumeration operation may not execute.'然而，MSDN 告訴我們SynchronizedReadOnlyCollection 類(lèi)提供一個(gè)線程安全的只讀集合，其中包含泛型參數(shù)指定類(lèi)型的對(duì)象作為元素。
1 回答
MYYA
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錯(cuò)誤的根本原因是List<T>構(gòu)造不是線程安全的。
讓我們看看在構(gòu)造 new 時(shí)會(huì)發(fā)生什么SynchronizedReadOnlyCollection。異常發(fā)生在以下行：
return new SynchronizedReadOnlyCollection<int>(testlist.SyncRoot, testlist);
正如異常 StackTrace 告訴我們的那樣，List<T>..ctor在構(gòu)建過(guò)程中涉及到：
at System.Collections.Generic.SynchronizedCollection`1.CopyTo(T[] array, Int32 index)
at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
at System.Collections.Generic.SynchronizedReadOnlyCollection`1..ctor(Object syncRoot, IEnumerable`1 list)
構(gòu)造函數(shù)中的以下片段List<T>顯示了錯(cuò)誤發(fā)生的位置。代碼是從MS 參考源中復(fù)制的，我清理了代碼中不需要的部分以便于閱讀。請(qǐng)注意，在注釋 (1) 和 (2) 之間還有其他線程在操作集合：
public List(IEnumerable<T> collection) {
ICollection<T> c = collection as ICollection<T>;
// (1) count is now current Count of collection
int count = c.Count;
// other threads can modify collection meanwhile
if (count == 0)
{
_items = _emptyArray;
}
else {
_items = new T[count];
// (2) SynchronizedCollection.CopyTo is called (which itself is thread-safe)
// Collection can still be modified between (1) and (2)
// No when _items.Count != c.Count -> Exception is raised.
c.CopyTo(_items, 0);
_size = count;
}
}
解決方案
testlist在構(gòu)造 new 時(shí)，可以通過(guò)鎖定修改輕松解決該問(wèn)題SynchronizedReadOnlyCollection。
public SynchronizedReadOnlyCollection<int> pubReadOnlyProperty
{
get
{
lock (testlist.SyncRoot)
{
return new SynchronizedReadOnlyCollection<int>(testlist.SyncRoot, testlist);
}
}
}
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