Page_Load中两次无序调用的方法 [英] Method called twice out of order in Page_Load

问题描述

我在SharePoint Stack Exchange中进行了询问，但认为这可能不是SharePoint特定的问题，并且可能与.NET页面生命周期有很大关系.原始问题可以在此处找到.

I asked this over in the SharePoint Stack Exchange but figured it might not be a SharePoint-specific question and might have more to do with the .NET page lifecycle. The original question can be found here.

我正在为SharePoint 2013编写一个Web应用程序，并且遇到了一些有趣的行为.基本上，我要进行一系列Web请求，但首先需要将它们存储在 Dictionary 中，以备后用.但是，如果在调试时打开3个选项卡并同时单击它们，我会发现 Dictionary 对象没有被清空，并且在尝试多次添加同一终结点时会导致异常.这是应用程序的相关代码:

I am writing a web app for SharePoint 2013 and am running into some interesting behavior. Basically, I am making a series of web requests, but first need to store those in a Dictionary for use later. However, if I open 3 tabs while debugging and hit them at the same time, I see the Dictionary object is not emptied and causes an exception when it tries to add the same endpoint multiple times. Here is the relevant code of the app:

public partial class TestControl : UserControl
{
    protected static Dictionary<string, string> _endpoints = new Dictionary<string, string>();

    protected void Page_Load(object sender, EventArgs e)
    {
        //clear the lists of endpoints each time the page is loaded
        _endpoints.Clear();
        ...
        MethodThatAddsToDictionary();
       ...
    }

    public static void MethodThatAddsToDictionary()
    {
        ...
        _endpoints.Add(response.First(), response.Last());
    }
}

调试，有时 MethodThatAddsToDictionary()在 _endpoints.Clear()在顶部的 Page_Load 事件下运行之前被调用两次，然后我将得到一个 ArgumentException :

Debugging, sometimes MethodThatAddsToDictionary() is called twice before the _endpoints.Clear() is run at the top under the Page_Load event and I'll get an ArgumentException saying:

具有相同键的项目已添加

an item with the same key has already been added

我觉得我缺少有关该应用程序生命周期的一些基本知识，但到目前为止还没有发现任何可行的方法.我可以在有条件的条件下包装 .Add()来检查密钥，然后再添加它，但是我觉得这是一个创可贴.我想念什么?

I feel like I'm missing something basic about the lifecycle of the app but haven't found anything that works so far. I could wrap the .Add() in a conditional to check for the key before I add it, but I feel like that is a bandaid. What am I missing?

提前谢谢！

推荐答案

ehhmm ... 如果您不将静态字典用作一种共享内存缓存，则可以将示例示例中的所有静态关键字都删除.在这种情况下，您也不需要调用 Clear()方法.

如果您将 static 词典用作一种内存缓存(我不建议这样做)，则您可能会遇到对页面的多次调用会导致竞争的情况，因为它们可能是由不同的线程处理.为了解决这个问题，您可以作为一个创可贴，使用lock语句在对字典的非线程安全调用上强制执行线程同步，如下所示:

If you use your static dictionary as a sort of memory cache, which I wouldn't recommend, you might experience that multiple calls to your page will result in a race condition, since they might be handled by different threads. In order to fix this, you can, as a band-aid, use a lock statement to force thread synchronization on the non-thread-safe call to the dictionary like this:

public partial class TestControl : UserControl
{
    protected static Dictionary<string, string> _endpoints = 
                                      new Dictionary<string, string>();

    //lock object needs to be static in this case
    private static object _lockObject = new object();

    protected void Page_Load(object sender, EventArgs e)
    {
        lock(_lockObject) //only one thread may pass at the same time,
                          //others will wait.
        {
            //clear the lists of endpoints each time the page is loaded
            _endpoints.Clear();
            ...
            MethodThatAddsToDictionary();

         }
     }

     public static void MethodThatAddsToDictionary()
     {
         ...
         _endpoints.Add(response.First(), response.Last());
     }
 }

请注意；这是一个肮脏的修补程序，将针对该特定调用有效地取消webs-server的所有多线程优化(从而导致性能下降).

Please note; this is a dirty fix and will effectively cancels all multithreading optimizations of the webs-server for this specific call (and thus causes a performance penalty).

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