使用的await时Monitor.Exit SynchronizationLockException

问题描述

我创建了一块code的，从遗留系统，我们有得到一个网页。为了避免过多的查询，我缓存得到的URL。我使用 Monitor.Enter ， Monitor.Exit 和双重检查，以避免请求发出两次，但是当释放与 Monitor.Exit 锁，我得到这个异​​常：

I am creating a piece of code that gets a webpage from a legacy system we have. In order to avoid excessive querying, I am caching the obtained URL. I am using Monitor.Enter, Monitor.Exit and double checking to avoid that request is issued twice, but when releasing the lock with Monitor.Exit, I am getting this exception:

System.Threading.SynchronizationLockException was caught
  HResult=-2146233064
  Message=Object synchronization method was called from an unsynchronized block of code.
  Source=MyApp
  StackTrace:
       at MyApp.Data.ExProvider.<OpenFeature>d__0.MoveNext() in c:\Users\me\Documents\Visual Studio 2013\Projects\MyApp\MyApp\Data\ExProvider.cs:line 56
    --- End of stack trace from previous location where exception was thrown ---
       at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
       at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
       at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
       at MyApp.Data.ExProvider.<GetSupportFor>d__15.MoveNext() in c:\Users\me\Documents\Visual Studio 2013\Projects\MyApp\MyApp\Data\ExProvider.cs:line 71
  InnerException: 

线56是 Monitor.Exit 。这是code执行操作：

The line 56 is the Monitor.Exit. This is the code that performs the operation:

private async Task<Stream> OpenReport(String report)
{
    var file = _directory.GetFiles(report+ ".html");
    if (file != null && file.Any())
        return file[0].OpenRead();
    else
    {
        try
        {
            Monitor.Enter(_locker);
            FileInfo newFile = new FileInfo(Path.Combine(_directory.FullName, report + ".html"));
            if (!newFile.Exists) // Double check
            {
                using (var target = newFile.OpenWrite())
                {
                    WebRequest request = WebRequest.Create(BuildUrl(report));
                    var response = await request.GetResponseAsync();
                    using (var source = response.GetResponseStream())
                        source.CopyTo(target);
                }
            }
            return newFile.OpenRead();
        }
        finally
        {
            Monitor.Exit(_locker);
        }
    }
}

那么，什么是问题伺机和显示器？是不是因为它是不一样的线程时 Monitor.Enter 比当 Monitor.Exit ？

So what is the problem with await and Monitor? Is it because it is not the same thread when Monitor.Enter than when Monitor.Exit?

推荐答案

您不能等待 A 锁内的任务范围（这是 Monitor.Enter 和 Monitor.Exit ）。使用显示器将直接骗过编译器而不是框架。

You can't await a task inside a lock scope (which is syntactic sugar for Monitor.Enter and Monitor.Exit). Using a Monitor directly will fool the compiler but not the framework.

异步等待有像显示器没有线程亲和力一样。在code中的后等待可能会在不同的线程比收到code运行。这意味着，释放显示器线程不一定是收购它的人。

async-await has no thread-affinity like a Monitor does. The code after the await will probably run in a different thread than the code before it. Which means that the thread that releases the Monitor isn't necessarily the one that acquired it.

要么不使用异步等待在这种情况下，或者使用不同的同步建设，比如 SemaphoreSlim 或一个 AsyncLock ，你可以建立你自己。这里是我的：

Either don't use async-await in this case, or use a different synchronization construct like SemaphoreSlim or an AsyncLock you can build yourself. Here's mine: http://stackoverflow.com/a/21011273/885318

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