ConcurrentDictionary apparently not working in ASP.NET controller - asp.net-core

Question

Why is the concurrent dictionary not behaving like a concurrent dictionary?
[Route("aaa")]
[HttpPost]
public Outbound Post(Inbound inbound)
{
Outbound outbound = new Outbound();
TaskState taskState = new TaskState();
taskState.state = 0;
TaskState s;
bool rc = taskDictionary.TryGetValue(inbound.taskId, out s);
if (rc == false)
taskDictionary.GetOrAdd(inbound.taskId, taskState);
else
return null;
if (s != null)
return null;
rc = taskDictionary.TryGetValue(inbound.taskId, out s);
if (rc)
{
string m0 = String.Format("POST dictionary count is " + taskDictionary.Count);
_logger.Log(LogLevel.Error, m0);
}
else
return null;
_logger.Log(LogLevel.Error, "POST method is doing long running work.");
long Billion = 1000000000;
// work quantity 5 means about 26 seconds // 30 billion is 215 seconds
for (long i = 0; i < inbound.WorkQuantity * Billion; i++)
;
outbound.Message = "The server did some work.";
outbound.BigObject = Modify(inbound.BigObject);
_logger.Log(LogLevel.Error, "POST finished long running work and will soon remove from the dictionary.");
rc = taskDictionary.Remove(inbound.taskId, out s);
if (rc == false)
return null;
if (s == null)
return null;
_logger.Log(LogLevel.Error, "POST is returning the object.");
return outbound;
}
[Route("bbb")]
[HttpPost]
public TaskState PostState(TaskId taskId)
{
TaskState s;
if (taskDictionary.TryGetValue(taskId, out s))
{
_logger.Log(LogLevel.Error, "POSTSTATE, state is " + s.AsString());
return s;
}
else
{
_logger.Log(LogLevel.Error, "POSTSTATE, state not found, dictionary count is " + taskDictionary.Count);
return null;
}
}
TaskDictionary taskDictionary = new TaskDictionary();
class TaskDictionary : ConcurrentDictionary<TaskId, TaskState>
{
internal bool IncrementState(TaskId taskId)
{
TaskState s;
if (TryGetValue(taskId, out s))
{
s.Increment();
return true;
}
else
return false;
}
}
The logging output is...
12.207 +00:00 [Error] MyController: POST dictionary count is 1
12.322 +00:00 [Error] MyController: POST method is doing long running work.
14.361 +00:00 [Error] MyController: POSTSTATE, state not found, dictionary count is 0
40.452 +00:00 [Error] MyController: POST finished long running work and will soon remove from the dictionary.
40.569 +00:00 [Error] MyController: POST is returning the object.
Specifically the problem is that request for the state information does not work because the dictionary appears empty.

Answer 1

So, based on the logging output you are doing two post requests to generate the output that you show us.
The way you initialize the Concurrent Dictionary, is the reason why it returns null on the second request. And that has to do with how the app receives those requests. Each request to your app is independent of another request.
Simply put;
A user POST's to you app.
The requests goes through the middleware pipeline, and eventually it will end up at your controller. Now, here's the important part, the controller will be 'constructed' for this particular request. And it will live for the duration of the request. That's because the lifetime of a controller is scoped by default. So the next request, will construct a new controller and that means that the Dictionary is different from the first one.
So to overcome this scoped request problem, you create a service that contains the dictionary, register this as a singleton (meaning it will only be constructed once and then it's shared), and use dependency injection to use it in the controller.