Xtykutl

I am trying to get the links from VSO for workitems, and the code seems to work fine when called from a single thread, but throws exceptions when called from a parallel for each loop.

I initalize my vso client object in my class's constructor:

vso = new WorkItemReporting(Config.VSTSAccessToken);

Then later in a method:

Parallel.ForEach(msrcAbBugsToProcess, new ParallelOptions { MaxDegreeOfParallelism = 10 }, bugId =>

{

    var workItemLinks = vso.GetWorkItemSourceCodeLinks(bugId);

});

witclient below is a WorkItemTrackingHttpClient which is in a different class (WorkItemReporting) and calls the API. It is this call that fails.

public List<string> GetWorkItemSourceCodeLinks(int bugId)

{

    var workItemSourceCodeLinks = new List<string>();         



    var workItem = _witClient.GetWorkItemAsync(bugId, null, null, WorkItemExpand.Relations).Result;

    if (workItem?.Relations != null)

    {

        var validSourceCodeLinkTypes = new List<string> { "ArtifactLink", "Hyperlink" };

        foreach (var relation in workItem.Relations)

        {

            if (validSourceCodeLinkTypes.Contains(relation.Rel))

            {

                workItemSourceCodeLinks.Add(relation.Url);

            }

        }

    }

}   

This works fine if I don't use the Parallel.ForEach and I get the needed data from the API. When I do, I get this exception 50% of the time:

Object reference not set to an instance of an object.   

at System.Security.Cryptography.X509Certificates.X509CertificateCollection.GetHashCode()

   at System.Net.HttpWebRequest.GetConnectionGroupLine()

   at System.Net.HttpWebRequest.SubmitRequest(ServicePoint servicePoint)

   at System.Net.HttpWebRequest.BeginGetResponse(AsyncCallback callback, Object state)

   at System.Net.Http.HttpClientHandler.StartGettingResponse(RequestState state)

   at System.Net.Http.HttpClientHandler.StartRequest(Object obj)

--- End of stack trace from previous location where exception was thrown ---

   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)

   at Microsoft.VisualStudio.Services.Common.VssHttpMessageHandler.<SendAsync>d__17.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at Microsoft.VisualStudio.Services.Common.VssHttpRetryMessageHandler.<SendAsync>d__4.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.<SendAsync>d__48.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.<SendAsync>d__45`1.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.<SendAsync>d__27`1.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.<SendAsync>d__26`1.MoveNext()

Is there something I'm doing wrong?

A solution is to create WorkItemReporting inside Parallel.ForEach:

Parallel.ForEach(msrcAbBugsToProcess, new ParallelOptions { MaxDegreeOfParallelism = 10 }, bugId =>

{

    var vso = new WorkItemReporting(Config.VSTSAccessToken);

    var workItemLinks = vso.GetWorkItemSourceCodeLinks(bugId);

});

You need to lock access to the external resource by doing something like the following.

As per comment @AlexSikilinda _witClient must not be thread safe.

For what your doing though, just because you can create multiple threads doesn't means you should, i would get timings, on what your doing with and without the parallel processing in your code,

Keep in mind if you use non thread safe external resources, and locks then it maybe just as slow as running in a normal for loop. You need to test both scenario's.

Add an object outside your methods to allow access to the lock object from every thread, this will help manage the access to the non thread safe class.

vso = new WorkItemReporting(Config.VSTSAccessToken);

private readonly object _witLock = new object();

In the following you DON'T want to put the lock around the entire methods contents as i have, otherwise you are definitely wasting your time using the Parallel for this piece of code.

Based on what your doing thought this should stop the exceptions.

You need refactor the code so that only the call to _witClient is within the lock. So just declare the workItem variable outside the lock with it proper type, and then only wrap the call to workItem = _witClient.GetWorkItemAsync(bugId, null, null, WorkItemExpand.Relations).Result; in your lock code.

public List<string> GetWorkItemSourceCodeLinks(int bugId)

{

    var workItemSourceCodeLinks = new List<string>();         

    lock (_witLock)

    {

        var workItem = _witClient.GetWorkItemAsync(bugId, null, null, WorkItemExpand.Relations).Result;

        if (workItem?.Relations != null)

        {

            var validSourceCodeLinkTypes = new List<string> { "ArtifactLink", "Hyperlink" };

            foreach (var relation in workItem.Relations)

            {

                if (validSourceCodeLinkTypes.Contains(relation.Rel))

                {

                    workItemSourceCodeLinks.Add(relation.Url);

                }

            }

        }

    }

}  

Good Luck