Task.WaitAll is throwing an Exception in rare cases

raj kiran

12y16kC#

HI dear members,

i am using an API to get data and store it in Database, the problem is, the API return only 50 records per a call, so to get more data in less time, we have used Parallel.For in the application, the code which is am using below is saving the data in database successfully most of the times, but it is throwing an exception in rare cases, the exception which it is throwing at rare cases is , The tasks array included at least one null element. Parameter name: tasks | at System.Threading.Tasks.Task.WaitAll(Task[] tasks, Int32 millisecondsTimeout, CancellationToken cancellationToken) at System.Threading.Tasks.Task.WaitAll(Task[] tasks, Int32 millisecondsTimeout).

I dont know the cause of the exception, because most of the time it is giving the correct results and i can save data in database, below is the code which i have written , please help me in resolving this issue, i am new to the parallel programing

object objobject = new object();

public void GetDataConcurrent()

{

var tasks = new List();

ParallelOptions objParallelOptions = new ParallelOptions();

CancellationTokenSource objCancellationTokenSource = new CancellationTokenSource();

objParallelOptions.MaxDegreeOfParallelism = 3;

objParallelOptions.CancellationToken = objCancellationTokenSource.Token;

string CompletePostData = "PostData to send to API";

string tException = string.Empty;

int noOfrequests = 15;

string jsonResponse=string.Empty;

try

{

Parallel.For(0, noOfrequests, objParallelOptions, (ChangeSetValue) =>

{

tasks.Add(Task.Factory.StartNew(() =>

{

var webRequest = WebRequest.Create("http://urlofAPI");

webRequest.ContentType = "application/json";

webRequest.Method = "POST";

webRequest.Proxy = WebRequest.DefaultWebProxy;

webRequest.Proxy.Credentials = CredentialCache.DefaultCredentials;

POSTDATA objPOSTDATA = new POSTDATA();

objPOSTDATA.CompletePostData = CompletePostData + ",'ChangeSetValue':'" + (ChangeSetValue) + "'}";

string json = new System.Web.Script.Serialization.JavaScriptSerializer().Serialize(objPOSTDATA);

var datatoSend = Encoding.UTF8.GetBytes(json);

webRequest.GetRequestStream().Write(datatoSend, 0, datatoSend.Length);

webRequest.GetReponseAsync().ContinueWith(t =>

{

if (t.Exception == null)

{

using (var sr = new StreamReader(t.Result.GetResponseStream()))

{

lock (objobject)

{

string str = sr.ReadToEnd();

jsonResponse = jsonResponse + str;

jsonResponse = jsonResponse.Replace("", string.Empty);

jsonResponse = jsonResponse.Substring(1, jsonResponse.Length - 2);

SaveData();// Method to Save Data in Database by Deserializing the JSON Response

jsonResponse = string.Empty;

}

else

{

tException = t.Exception.InnerException.Message;

}

}).Wait();

}));

});

Task.WaitAll(tasks.ToArray());

}

catch (OperationCanceledException ex)

{

LogException(InsertLogInformation(ex.Message));

}

catch (AggregateException ex)

{

for (int j = 0; j < ex.InnerExceptions.Count; j++)

{

LogException(InsertLogInformation(ex.Message));

}

catch (Exception ex)

{

LogException(InsertLogInformation(ex.Message));

}

public void SaveData()

{

//Method to Save Data in DataBase

}

public static class WebRequestExtensions

{

public static Task GetReponseAsync(this WebRequest request)

{

return Task.Factory.FromAsync(request.BeginGetResponse, request.EndGetResponse, null);

}

public class POSTDATA

{

public string CompletePostData { get; set; }

}

5 Replies

Know the answer? Post it — somebody with the same question will find it here.

Benjamin Scharbach9y ago
It sounds like the call to the Task.Factory in your code might not correctly return a Task instance, for whatever-reason. Therefore, inside the Parallel.For call, I would recommend creating the Task.Factory call to return into a var, then check if the var is null before adding to your Task array.

Ben Scharbach
https://www.youtube.com/channel/UCih2aBHyGR5cJHApqkwAXBA
Alan Lu11y ago

It is a timing issue
Parallel.For(0, noOfrequests, objParallelOptions, (ChangeSetValue) =>
{
tasks.Add(Task.Factory.StartNew(() =>
{
The above line is the issue
You should not be using Parallel.For to populate the list of tasks since Parallel.For itself may/will create threads which will access a shared collection "tasks" to populate it
To reproduce the issue, add a Thread.Sleep(random) above the tasks.Add(Task.Factory.StartNew(() =>
In some cases, it takes longer to create a task itself thus making one of the values in the "tasks" collection NULL.
Vulpes12y ago
AFAICS, your code is fine.

The mystery is how you can possibly get a null Task in your Tasks list when you're using Task.Factory.StartNew() to create them. This method is supposed to call the Task constructor before it does anything else so it's difficult to see how it could ever return null.
raj kiran12y ago
Hi Vulpes, thanks for your response, as suggested by you i have added the code you have mentioned. but can you tell me, the code which i have written for the getting data from API using Parallel.For and tasks is correct way of doing, or is there any better approach for implementing concurrency.
Vulpes12y ago
I've no idea what could cause some tasks to be null but, as a preventative measure, I'd suggest removing all null tasks before you call the Task.WaitAll method. In other words, replace this line:

Task.WaitAll(tasks.ToArray());

with these:

Task[] tasksArray = tasks.Where(t => t != null).ToArray();
if (tasksArray.Length > 0) Task.WaitAll(tasksArray);