从我的Web API调用第三方Web API时，Web服务器挂起

Question

I need to call a 3 party Web Api from my ASP.net Web API, when it runs the line

HttpResponseMessage response = await client.PostAsJsonAsync("api/ArTrx/postDocument", poMaster);

the web server hangs. I have tried to put below code in to a console program, it runs well. What is wrong with below code when run in ASP.net Web API project?

client.BaseAddress = new Uri(webAPIAddress);
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

string token = Crypto.EncryptStringAES("abcd1234");
client.DefaultRequestHeaders.Add("X-Token", token);

HttpResponseMessage response = await client.PostAsJsonAsync("api/ArTrx/postDocument", poMaster);    
if (response.IsSuccessStatusCode)
{
    ...
}

Answer 1

You probably have a Task<T>.Result or Task.Wait call further up your call stack. This will cause a deadlock on ASP.NET that I describe in more detail on my blog.

In summary, await will capture a "context" - in this case, an ASP.NET request context, which only allows one running thread at a time. When its awaitable completes (that is, when the POST finishes), the async method will continue in that context . If your code is blocking further up the call stack (eg, Result or Wait ), then it will block a thread in that ASP.NET request context, and the async method cannot enter that context to continue executing.

You don't see the deadlock in the Console app because there's no context being captured, so the async method resumes on a thread pool thread instead.