WebAPI HttpContext is Null Inside ContinueWith() => tast - asp.net-web-api

Question

I'm just wondering if someone could explain what is happening here.
Given this Post method on an API controller:
public HttpResponseMessage PostImage()
{
var request = HttpContext.Current.Request;
var c = SynchronizationContext.Current;
var result = new HttpResponseMessage(HttpStatusCode.OK);
if (Request.Content.IsMimeMultipartContent())
{
Request.Content.ReadAsMultipartAsync(new MultipartMemoryStreamProvider()).ContinueWith((task) =>
{
MultipartMemoryStreamProvider provider = task.Result;
foreach (HttpContent content in provider.Contents)
{
Stream stream = content.ReadAsStreamAsync().Result;
Image image = Image.FromStream(stream);
var uploadFileName = content.Headers.ContentDisposition.FileName;
var requestInside = HttpContext.Current.Request; // this is always null
string filePath = Path.Combine(HostingEnvironment.MapPath(ConfigurationManager.AppSettings["UserFilesRootDir"]), userprofile.UserCode);
//string[] headerValues = (string[])Request.Headers.GetValues("UniqueId");
string fileName = userprofile.UserCode + ".jpg";
string fullPath = Path.Combine(filePath, fileName);
image.Save(fullPath);
}
});
return result;
}
}
Why would var requestInside = HttpContext.Current.Request; be null?
I've checked all the relevant settings:
<compilation debug="true" targetFramework="4.5">
...
<httpRuntime targetFramework="4.5"
And SynchronizationContext.Current is the newer AspNetSynchronizationContext rather than LegacyAspNetSynchronizationContext.
I'm presuming at the moment that it's because I'm on a different thread, is this a correct assumption?

Answer 1

ContinueWith is not guaranteed to run on the same thread hence the synchronization context could be lost. You could change your call to specify to resume on the current thread with parameter TaskScheduler.Current. See this previous SO answer.
If you use await/async pattern it will automatically capture the current syncronization context on resume once an awaitable operation completes. This is done by resuming the operation on the same thread which is bound to that context. An added benefit, IMHO, is cleaner looking code.
You can change your code to this which uses that pattern. I have not made any other changes to it other than use async/await.
public async Task<HttpResponseMessage> PostImage()
{
var request = HttpContext.Current.Request;
var c = SynchronizationContext.Current;
var result = new HttpResponseMessage(HttpStatusCode.OK);
if (Request.Content.IsMimeMultipartContent())
{
MultipartMemoryStreamProvider provider = await Request.Content.ReadAsMultipartAsync(new MultipartMemoryStreamProvider());
foreach (HttpContent content in provider.Contents)
{
Stream stream = await content.ReadAsStreamAsync();
Image image = Image.FromStream(stream);
var uploadFileName = content.Headers.ContentDisposition.FileName;
var requestInside = HttpContext.Current.Request; // this is always null
string filePath = Path.Combine(HostingEnvironment.MapPath(ConfigurationManager.AppSettings["UserFilesRootDir"]), userprofile.UserCode);
//string[] headerValues = (string[])Request.Headers.GetValues("UniqueId");
string fileName = userprofile.UserCode + ".jpg";
string fullPath = Path.Combine(filePath, fileName);
image.Save(fullPath);
}
}
return result;
}