REST API code snippet throws NullReferenceException in ASP.NET web page while running fine in WinForms

Question

Using the following code snippet in WinForms, I am successfully able to upload documents through a REST API. But as soon as I move to ASP.NET, it throws a NullReferenceException.

public async Task<string> TestUpload()
    {
        const string cServerBaseAddress = "https://test-testrestservice.abcd.com/";
        const string cFilename = @"D:\temp\Test.txt";
        const string cUrl = "{0}abc/dms/api/v1/crmdocuments/?BusinessUnit={1}&AccountID={2}&DocumentType={3}&Description={4}&Filename={5}";
        string businessUnit = "XYZ";
        string accountID = "ABCCompany";
        string docType = "Affidavit";
        string description = @"\%&description&%/";
        string responseContent = string.Empty;
        try
        {
            string url = string.Format(cUrl, cServerBaseAddress,
            WebUtility.UrlEncode(businessUnit), WebUtility.UrlEncode(accountID),
            WebUtility.UrlEncode(docType), WebUtility.UrlEncode(description),
            WebUtility.UrlEncode(Path.GetFileName(cFilename)));
            using (HttpClient client = GetClient(cServerBaseAddress))
            {
                using (HttpResponseMessage response = await client.PostAsync(url, new ByteArrayContent(File.ReadAllBytes(cFilename))))
                {
                    responseContent = await response.Content.ReadAsStringAsync();
                    response.EnsureSuccessStatusCode();
                    string newContentId = Newtonsoft.Json.Linq.JToken.Parse(response.Content.ReadAsStringAsync().Result).ToString();
                    return newContentId;
                }
            }
        }

I've debugged the client, url, and ByteArrayContent, and none of them is null. But I am still getting an unhandled exception.

Here are the details of the exception:

   at System.Web.ThreadContext.AssociateWithCurrentThread(Boolean setImpersonationContext)
   at System.Web.HttpApplication.OnThreadEnterPrivate(Boolean setImpersonationContext)
   at System.Web.LegacyAspNetSynchronizationContext.CallCallbackPossiblyUnderLock(SendOrPostCallback callback, Object state)
   at System.Web.LegacyAspNetSynchronizationContext.CallCallback(SendOrPostCallback callback, Object state)
   at System.Web.LegacyAspNetSynchronizationContext.Post(SendOrPostCallback callback, Object state)
   at System.Threading.Tasks.SynchronizationContextAwaitTaskContinuation.PostAction(Object state)
   at System.Threading.Tasks.AwaitTaskContinuation.RunCallback(ContextCallback callback, Object state, Task& currentTask)
--- End of stack trace from previous location where exception was thrown ---
   at System.Threading.Tasks.AwaitTaskContinuation.<>c.<ThrowAsyncIfNecessary>b__18_0(Object s)
   at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

Null exception on what specifically? It could be that you are missing the url and or a config setting. — Jaya
– Jaya, Commented Feb 19, 2018 at 17:42
I checked I've none of them is null or empty. I've also updated the question with stack trace — J. Doe
– J. Doe, Commented Feb 19, 2018 at 17:43
A few points that might not fix the specific issue: a) call EnsureSuccessStatusCode() before you try to read the content; b) you don't declare responseContent in this code, so where is it declared?; c) it could be this is just an adjusted code snippet, but in general do not create and dispose HttpClient like that for each request - create one instance and use it for your whole app, as intended; d) change response.Content.ReadAsStringAsync().Result to just responseContent. — sellotape
– sellotape, Commented Feb 19, 2018 at 18:12
@sellotape this is the part of whole program to upload document through api. its almost more than 1500 lines of code, if you wish I would update here as well — J. Doe
– J. Doe, Commented Feb 19, 2018 at 18:17
@J.Doe - I don't think anyone wants (or will read) 1500 lines of code in a SO question :) Have you tried Michael Liu's answer, as well as running EnsureSuccessStatusCode() first? Could be you're getting a non 2xx response. — sellotape
– sellotape, Commented Feb 19, 2018 at 18:22

Michael Liu · Accepted Answer · 2018-02-20 19:00:14Z

1

The presence of "LegacyAspNetSynchronizationContext" in your stack trace is a clue as to what's wrong.

If you want to use await in ASP.NET 4.5, then you must add the following element in Web.config:

<appSettings>
    <add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />
</appSettings>

From the documentation (emphasis added):

Setting this compatibility switch is mandatory for WebSockets-enabled applications, for using Task-based asynchrony in Web Forms pages, and for certain other asynchronous behaviors.

UPDATE: If you want to call an async method from a non-async method, and you don't care about the result, then one way to do so is to use Task.Run:

Task.Run(() => TestUpload());

edited Feb 20, 2018 at 19:00

answered Feb 19, 2018 at 17:50

Michael Liu

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8 Comments

J. Doe Over a year ago

Thanks Mich, I added that snippet to <configuration>. Now, I am not facing the exception but the breakpoint is not hitting back to the last block of code.

Michael Liu Over a year ago

Edit your post and show how you're calling this code snippet. You might be encountering a deadlock (which could happen if you're accessing the Result property of a Task).

J. Doe Over a year ago

This function is put in one function, and the whole function is called all at once. I've updated the code as well

J. Doe Over a year ago

it is called upon a button click, TestUpload();

Michael Liu Over a year ago

Are you awaiting the result, or accessing the Task's Result property?

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