How to use HttpClient in WCF syncronously

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I am trying to post Multipartformdatacontent from my proxy wcf service, but it behaves differently when I use PostAsync method from HttpClient. WCF is not waiting for result to come back, instead its executing next line of code immediately and failing due to object reference error. Same solution works in console application. Any reason ? am I missing any threading model in WCF

C#

var message = new HttpRequestMessage();
message.Method = HttpMethod.Post;
message.Content = content;
message.Headers.Add("VSAML", vSaml);
message.RequestUri = new Uri(cmsStoreUrl);
 
var client = new HttpClient() { Timeout = TimeSpan.FromMinutes(15) };
var response = client.PostAsync(cmsStoreUrl, content);
var res = response.Result.EnsureSuccessStatusCode();
var a = res.Content.ReadAsStringAsync();
cmsStoreResponse = a.Result;

As soon as I do PostAsync, it throws null reference exception in next line. same code works in console application

Posted 31-Jul-14 7:38am

sam3440

Updated 1-Aug-14 6:14am

v4

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Sergey Alexandrovich Kryukov 31-Jul-14 14:02pm

Probably not; it could be just some bug. To discuss it, you would need to create some complete but short code sample manifesting the problem. Please see: SSCCE.
—SA

sam3440 31-Jul-14 16:41pm

Not sure if I need to report as bug, I did not find any one having same issue. So that makes me think that HttpClient should be used in different way in WCF ? any thought

Sergey Alexandrovich Kryukov 31-Jul-14 23:52pm

What thought? Why would some class would behave differently? Anyway, you really need to show some code. So far, you did not provide any information useful for resolution...
—SA

sam3440 1-Aug-14 10:10am

Updated code in question.

As soon as I do PostAsync, it fails in next line. same code works in console application

Sergey Alexandrovich Kryukov 1-Aug-14 11:59am

First of all, please, code goes to the question, using "Improve question". Format if (in the tag <pre lang="cs">...).
What does it mean, fail? what line? full exception information, please.
—SA

sam3440 1-Aug-14 12:15pm

it throws null reference exception for Result property in response.

full exception.System.AggregateException: One or more errors occurred. ---> System.NullReferenceException: Object reference not set to an instance of an object.

Sergey Alexandrovich Kryukov 1-Aug-14 20:54pm

What is null, response or response.Result?
—SA

sam3440 1-Aug-14 21:28pm

response.Result is null

Sergey Alexandrovich Kryukov 1-Aug-14 21:48pm

Your problem looks clear to me; please see my answer.
—SA

Kerwen Zhang 29-Mar-18 6:02am

Hi sam, have your problem been resovled? I met a similar problem now. I use SendAsync to send a http request, the method work in cosole application but return null in WCF. May I know your solution?

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Sergey Alexandrovich Kryukov · Answer 1 · 2014-08-01T15:48:00

This is asynchronous operation! You should not assume that the result is ready immediately. For a while response.Result will remain null, and later it might turn to real result.

So, the conclusion is: in both cases, in your console-only application and WCF application, this fragment of code was wrong, really badly wrong. The fact it ran in console application is even worse: it looks like the only difference is timing. You created a bad thing, incorrect dependency on the order of execution, usually called race condition: http://en.wikipedia.org/wiki/Race_condition[^].

The readiness of the result of asynchronous operation and check up of this result raced again each other; if the result was ready sooner, it created only the illusion of success.

For asynchronous operation usage, please see:
http://msdn.microsoft.com/en-us/library/system.net.http.httpclient.postasync%28v=vs.118%29.aspx[^],
you used this form of it: http://msdn.microsoft.com/en-us/library/hh138190%28v=vs.118%29.aspx[^].

For correct use of asynchronous result, please see, for example, this code sample: http://www.asp.net/web-api/overview/web-api-clients/calling-a-web-api-from-a-net-client[^].

Now, for service operation (and many other cases), I usually recommend using synchronous (blocking) API in combination with explicitly created separate threads. Please see my arguments:
problem in multithreading ? !!![^],
TcpListener, TcpClient, Ping in Visual Basic 2005[^],
TCP Socket - Send and receive question[^],
Async Await Multiple Telnet Servers[^].

—SA