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ASP.NET Web Api, Database connection in Threads

I have an issue with using a Database in a thread in my asp.net Application.
When I want to start my application I want to start a thread called "BackgroundWorker" with it, which runs in the background till the whole application is stopped.
The problem is that I have massive problems with the dbContext in the thread.
I I try to start the walker in my Startup.cs in the methods "ConfigureServices" or "Configure" and then initialize the dbContext in the constructor in the Walker like this "dbContext = new ApplicationContext()" it tells me that the connection is not configured, when I try to operate in the while(true) queue on the database.
If I write an own Controller for the Walker which receives a ApplicationContext in his constructor and then starts a Thread like this, if i call this controller once with a GET Request:
public BackgroundWorker(ChronicusContext dbContext)
{
_dbContext = dbContext;
_messageService = new MailMessageService();
}
// GET: api/backgroundworker
[HttpGet]
[Route("start")]
public void StartWorker()
{
//Thread thread = new Thread(this.DoBackGroundWork);
Thread thread = new Thread(() => DoBackGroundWork(this._dbContext));
thread.Start();
}
public void DoBackGroundWork(ChronicusContext _dbContext)
{
while (true)
{
if (_dbContext.PollModels.Any()) //Here is the exception
{
...
}
}
}
Then I receive an System.ObjectDisposedException that the object is already disposed inside the while (true) queue.
I tried those and similar things in many different ways but allways receive exceptions like these two or that the database connection is closed.
Can somebody help me and tell me, how this works?
Thank you!

Generally, server side multithreading for Web Applications does not happen often and is, most times, a huge no no.
Conceptually, your server is "multithreaded", it handles many HTTP requests from clients/users/other servers. For mobile and web architecture/design, your server(s) process multiple requests and your clients are handling asynchronous calls and dealing with waiting for responses from long running calls like your API method StartWorker.
Think of this scenario, you make a request to your WebAPI method StartWorker, the client, making the request is waiting for a response, putting the work on another thread does nothing as the client is still waiting for a response.
For example, let's consider your client an HTML web page with an Ajax call. You call StartWorker via Ajax, you will be loading data into a HTML table. You will desire, from a UX perspective, to put up a progress spinner while that long running StartWorker responds to your HTML Page Ajax call request. When StartWorker responds, the Ajax call loads the HTML table with the StartWorker response. StartWorker has to respond with the data. If StartWorker responds beforehand than you will have to send a push notification, via SignalR, for example, when the other thread completes and has the data you need for the HTML table.
Hopefully, you see, the call to the WebAPI method, takes the same amount of time from a Ajax request/response perspective, so multithreading becomes pointless in this scenario, a most common web application scenario.
You can have your client UI load other UI elements, showing a progress spinner in HTML table UI area, until your database call is complete and responds with the data to your Ajax call. This way your users know things are happening and something is still loading.
If you still need your additional thread in your API for your project needs, I believe you have to be using Entity Framework 6 or greater to support asynchronous queries, see this tutorial:
http://www.codeproject.com/Tips/805923/Asynchronous-programming-in-Web-API-ASP-NET-MVC
UPDATE
Now that I know you need to run a SQL query on a repeating frequency of time, and you have an Azure Web App, what you want to use is Azure Automation if you are using Sql Azure or create a Sql Server Job if you are using a Sql Server instance as your backend

DbContext is not thread safe. You need to create a new context from inside your thread.
public void DoBackGroundWork()
{
ChronicusContext anotherContext= new ChronicusContext();
while (true)
{
if (anotherContext.PollModels.Any())
{
...
}
}
}

ASP.NET WebAPI Ajax with progress

I have a really long webAPI request that basically does the follow :
1. retrieves a list of item categories from the db
2. for each category, retrieve all the items in the category
Now, the entire process takes a very long time and I don't want the user to wait till the entire process is over, if a category has finished loading I want it to return to the client
Does anyone know how I can do that? Send a request and get progress notifications by the server whenever a part of the request has finished?

You could use SignalR to send the data from the server to the client when it's available.
The other option is polling from the client. The client makes the initial request, which triggers a server side process that prepares the data and keeps it somewhere (in memory, in a database). Then the client polls the server for new available data until the server process finishes.

you need to break your request. use for loop. if elements from first category are downloaded then do something with them before going for second category.
so your request will go inside some loop. You can use Jquery or page methods if you are using asp.net webforms

PushStreamContent might help you:
http://weblogs.asp.net/andresv/asynchronous-streaming-in-asp-net-webapi

how to call javascript function at backend in asp.net?

If I want to call a javascript function after some code has been run in backend, how to do that? I don't want to add onclientclick event in frontend. I need to run the frontend function after a certain backend code has been run. Thanks in advance.

The best bet is to use Page.RegisterStartupScriptBlock.
When you submit to the server, the server is going to process your request and any other event / code that is required. It will then send back to the client the response.
If you want a user to invoke an action / event (say a button click), have it hit the server and then do some Java Script action after that please look at the following posts:
http://forums.asp.net/t/1003608.aspx/1
http://wiki.asp.net/page.aspx/1574/differences-between-registerclientscriptblock-amp-registerstartupscript-and-how-they-work-with-ajax-update-panel/
http://www.codeproject.com/KB/aspnet/ClientServer.aspx

The way (I) would do this is to refactor my code so that the code behind is called from a jQuery post to an ashx code file.
then on return of that function i can run whatever I like.
If you want details, let me know.
However, if this is not useable, then you could either place a property in your aspx page;
public string Action { get; set; }
then in your html;
var action = '<%= this. Action %>';
and perform an action.
Or you might set a hidden field to a value and check against that.
edit
this largely depends on when in the life cycle the code behind is being run. On ajax postback, on initial execution? When?

Client Script in ASP.NET Web Pages is the MSDN reference that you probably want.

The best way i can think of to do this is with web sockets. There are a few c# web socket implimentations around, such as Nugget. This allows you to open a socket with your client and allows for the possibility of the server proactively contacting and sending data to the client. There are other techniques to have the server talk to the client such as long polling. This way you can have the server call the client once something has happened on the server.

Moving a time taking process away from my asp.net application

My Asp.net application generates a dynamic pdf. Sometimes this takes a while and is a quite heavy process. Actually i dont want my users to wait for the pdf, just send it to there mail after it generated.
So I tried a webservice. I'm passing an id (to get the data from the database) and some strings to the websercice's method.
But also with a webservice (even with asynchronous calls) the client only receives its response after the pdf is generated. So the user still has to wait.
So I'm kinda stuck, there must be a way i'm overlooking.

You don't need a webservice in order to get the ability to make asynchronous invocations.
You can just use ThreadPool.QueueUserWorkItem() as a fire-and-forget approach in the ASPX page, then return a reply with some sort of "work item id" - like a receipt or an order number.
Generate the PDF in the WaitCallback you pass to QUWI.
when the pdf is ready, that WaitCallback can send an email, or whatever.
Use a webservice if you want the function to be accessible, outside the webpage. Don't use it strictly for asynchrony.

Issue is that in your ASP.NET page code, you must be invoking the web service synchronously so the page waits till web service returns. You should try invoking the web service asynchronously (or on the different thread) and then don't wait for it to complete. Typically, visual studio generated proxy already has asynchronous overloads that you may use.
Alternately, you may modify your web service code - essentially, when request to your web method comes, you can start PDF generating on a different thread so that your web method may end indicating your client (page in this case) that request has been successfully scheduled for processing.

there are two ways which i know
First ways;
In asp.net code behind (in xxx.aspx.cs file) you can define a void method then you can call the method by starting a thread like below.
protected void SenMail(object prms)
{
int id = int.Parse(prms.ToString());
//mail sending proces
}
//starting SendMail method asynchronous
Thread trd = new Thread(SenMail);
trd.Start(idValue);
Second way;
You can create and mail sender page like "SendMail.aspx", then you can make an ajax request in javascript and no need to wait any response. you can pass id value to aspx page as request parameter.

Real-time updates like Twitter

How do you implement a real-time update like Twitter does ("423 more tweets since you started searching")? I need to refresh my page at least every 10 seconds. I'm using ASP.NET MVC and jquery.

You could use the setInterval javascript function which allows you to poll the server for updates at regular intervals using AJAX:
window.setInterval(function() {
// This will be executed each 5s
// TODO: query your server for updates probably using AJAX
// example with jquery:
$.getJSON('/arethereanyupdates', function(data) {
if (data.isupdates) {
alert('yeap there are updates');
}
});
}, 5000);
There's also a push technology in HTML5 called WebSockets which allows the server to notify the client for updates but of course you will need an HTML5 compatible browser which nowadays is not difficult to find and a WebSocket API compliant server.

You can do an AJAX call to your server asking for updates, if the answer is positive (there are new things) you can show a javascript windows advising the user or just update the page content you need.
regards.
UPDATE: This how-to implements similar behavior

In the absence of long-polling (aka COMET) support in todays browsers (this should, however, change soon, with HTML5), I'm polling the server every n seconds using AJAX request and sending the timestep of the latest visible entry/item/tweet/whatever. Then the backend count the number of newer items and returns it back to the browser.