在ASP.NET中的HttpClient调用

Question

I am facing problem in calling HttpClient class and async call. I call the function List() from page_load. The calls return from the line HttpResponseMessage response = await client.GetAsync(str); and never comes back to finish it.

I don't understand what mistake I am doing. Following is my code:

        protected void Page_Load(object sender, EventArgs e)
        {
            Task<string> s= List(product); 
        }

        protected async Task<string> List(string ProductType)
        {
            string str = "http://xx.xx.com/wiki/api.php";


            using (HttpClient client = new HttpClient())
            {
                client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", Convert.ToBase64String(Encoding.ASCII.GetBytes(String.Format("{0}:{1}", "username", "password"))));
                HttpResponseMessage response = await client.GetAsync(str);

                response.EnsureSuccessStatusCode();

                string content = await response.Content.ReadAsStringAsync();
            }
            return content;
         }

It never executes following lines.

response.EnsureSuccessStatusCode();
string content = await response.Content.ReadAsStringAsync();

Please help. Thanks in advance.

Answer 1

You never show what happens to the task in Page_Load , but given your symptoms and the fact that Page_Load is not async , I'm betting that you're calling Result or Wait instead of await ing it.

When you do that, you cause a deadlock . The link to my blog explains this in detail, but the gist is that await will capture and restore the current "context". In ASP.NET, this is a request context, and the request context only allows one thread executing in it at a time. So, if you call Result or Wait (while you're in the request context), you'll block a thread in that request context until the Task<string> completes. Meanwhile, when the HttpClient gets its response, it tries to resume after the await client.GetAsync(str) . But the request context will only allow one thread in at a time, and that thread is blocked waiting for the Task to complete. Since List cannot complete because the context is busy, you have a deadlock.

To prevent deadlocks, follow these two best practices (from my recent MSDN article) :

1. Use async all the way. That is, use await instead of Result or Wait .
2. Use ConfigureContext(false) in your "library" code, ie, every await in List should actually be await ... .ConfigureAwait(false);