僵局的WebAPI从异步DelegatingHandler异步读取响应内容 [英] Deadlock reading async response content from async DelegatingHandler in webapi

问题描述

问题

我使用的WebAPI一个DelegatingHandler读取响应的内容，并将其审计数据库。

I'm using a DelegatingHandler in WebAPI to read the response Content and audit it to the database.

当code到达.ReadAsStringAsync（）它似乎锁定和完成prevent其他请求。我只知道这一点，因为当我删除有问题的线，它完美的作品。

When the code gets to .ReadAsStringAsync() it appears to lock and prevent other requests from completing. I only know this, because when I remove the offending line, it works perfectly.

public static void Register(HttpConfiguration config)
{
    config.MessageHandlers.Add(new ResourceAuditHandler());
}

public class ResourceAuditHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,
                                                           CancellationToken cancellationToken)
    {
        var response = await base.SendAsync(request, cancellationToken);

        string content = "";
        if (response.Content != null)
        {
            //THIS SEEMS TO CREATE DEADLOCK!
            content = await response.Content.ReadAsStringAsync(); 
        }

        AuditResponseToDatabase(content);

        return response;
    }
}

可能的原因

我读过这可能发生在应用程序的两位试图读取相同的请求/响应的内容（DelegatingHandler和控制器/ ModelBinder的）...或者，当一个人试图读取更具体的在 。结果异步响应，但另一个实例已经阅读了结果/响应流中，然后空。

I've read this can happen when two bits of the application are trying to read the content from same request/response (a DelegatingHandler and Controller/ModelBinder)... Or more specifically when one tries to read the .Result of the async response, but the other instance has already read the Result / Response Stream, which is then null.

我的第一个要求，打一个简单的控制器，但第二个请求，击中一个控制器，检查行动参数ID，以检查它不是空的属性。我读过，当两个一DelegatingHandler和ModelBinder的正试图从你得到僵局响应阅读器。

My first request, hits a simple controller, but the second request, hits a controller with an attribute that checks the Action parameter "id" to check its not null. I've read that, when both a DelegatingHandler and the controller ModelBinder are trying to read from the Response you get deadlock.

替代方法（这也失败了）

我也尝试使用它使用的 .ReadAsByteArrayAsync（）其他的方法（如在其他做题所示），但这似乎仍然锁定。我明白，这种方法不使用。结果的办法，直接从响应流（不知）。

I've also tried using the other approach (as seen in other SO questions) which uses .ReadAsByteArrayAsync() but this still seems to lock. I understood that this method doesn't use the .Result approach and reads it directly from the response stream (somehow).

 var response = await base.SendAsync(request, cancellationToken);

 string content= "";
 if (response.Content != null)
 {
     var bytes = await response.Content.ReadAsByteArrayAsync();
     responseContent = Encoding.UTF8.GetString(bytes);
 }

 AuditResponseToDatabase(content);

请帮帮忙！

推荐答案

彼得，首先，你对这个僵局的原因。尝试使用异步/的await 一路下跌。换句话说，不要在阻止异步 code。你应该做的另一件事是使用 .ConfigureAwait（假）：

Peter, first of all, you are right about the cause of this deadlock. Try to use async/await all the way down. In other words, do not block on async code. Another thing you should do is using the .ConfigureAwait(false):

    var response = await base.SendAsync(request, cancellationToken).ConfigureAwait(false);

    string content = "";
    if (response.Content != null)
    {
        content = await response.Content.ReadAsStringAsync().ConfigureAwait(false); 
    }

更多信息，你可以找到在这个链接

如果没有上述工作，请提供一个完整的例子，好吗？

If none of the above work, please provide a complete example, ok?

希望这有助于！

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