List.Add()线程安全 [英] List.Add() thread safety

问题描述

我了解通常情况下，列表不是线程安全的，但是如果线程从不对列表执行任何其他操作(例如遍历列表)，则简单地将项目添加到列表中有什么错误吗?

I understand that in general a List is not thread safe, however is there anything wrong with simply adding items into a list if the threads never perform any other operations on the list (such as traversing it)?

示例:

List<object> list = new List<object>();
Parallel.ForEach(transactions, tran =>
{
    list.Add(new object());
});

推荐答案

在幕后发生了很多事情，包括重新分配缓冲区和复制元素.该代码将导致危险.很简单，添加到列表时没有原子操作，至少需要更新长度"属性，并且需要将项放在正确的位置，并且(如果有单独的变量)索引也需要要被更新.多个线程可以相互踩踏.如果需要增长，那么还有更多的事情要做.如果某项内容正在写入列表，则其他任何内容都不应读取或写入.

Behind the scenes lots of things happen, including reallocating buffers and copying elements. That code will cause danger. Very simply, there are no atomic operations when adding to a list, at the least the "Length" property needs to be updates, and item needs to be put in at the right location, and (if there's a separate variable) the index needs to be updated. Multiple threads can trample over each other. And if a grow is required then there is lots more going on. If something is writing to a list nothing else should be reading or writing to it.

在.NET 4.0中，我们有并发集合，它们很容易实现线程安全并且不需要锁.

In .NET 4.0 we have concurrent collections, which are handily threadsafe and don't require locks.

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