# Background Tasks Background Tasks are one of the most important features of Wisej.NET. No other Web development system allows developers to launch background tasks (threads) on the server and update the client asynchronously with the same level of simplicity and robustness. {% file src="" %} Download Wisej.BackgroundTasks.zip {% endfile %} ## What is a Background Task? A Background Task runs on the server in the background after the client request has completed, allowing users to continue working while the task executes. Background tasks operate independently from the session and user that initiated them. They run as separate threads without any session knowledge. You can even start separate threads when an application first loads, before any session or user request exists. {% hint style="info" %} To run code when an application first loads, create a static constructor on any class. The most appropriate would be `Program.cs` or the first page or window created for new sessions. {% endhint %} **What can I do in a Background Task?** Anything - connect to other servers, query databases, use the file system, access remote services, etc. **How do I terminate a Background Task?** Simply return from the call. A background task (like any thread) starts by calling your start method and runs until you exit it. Typically it's a loop that checks conditions and uses `Thread.Sleep(ms)` periodically, exiting when complete. ## Update the UI Wisej.NET provides a straightforward way to launch a Background Task bound to a specific session, allowing code to interact with the user's UI in real-time. {% tabs %} {% tab title="C#" %} ```csharp void button1_Click(EventArgs e) { Application.StartTask(() => { // Change the text of button 1 from 1 to 10 in the background. for (int i = 0; i < 10; i++) { this.button1.Text = i.ToString(); // We are running out-of-bound and we need to push the updates // to the client using Application.Update(). It can be called at any time // and interval. It simply flushes the changed back to the client. Application.Update(this); Thread.Sleep(1000); } }); } ``` {% endtab %} {% tab title="VB.NET" %} ```visual-basic Private Sub button1_Click(e As EventArgs) Application.StartTask(Sub() ' Change the text of button 1 from 1 to 10 in the background.' For i = 0 To 10 Me.button1.Text = i.ToString() ' We are running out-of-bound and we need to push the updates' ' to the client using Application.Update(). It can be called at any time' ' and interval. It simply flushes the changed back to the client.' Application.Update(this) Thread.Sleep(1000) Next End Sub) End Sub ``` {% endtab %} {% endtabs %} The example above updates the specific `button1` in the session that started the task because the thread is started using `Application.StartTask()`. `Application.StartTask()` starts a new thread and updates the context to match the calling session. If using `Thread.Start()` instead, the thread method couldn't access session objects unless wrapped in `Application.RunInContext()`. When code runs in context (either in-bound or out-of-bound for background tasks), you can call `Application.Update()` anytime to push UI updates to the client. {% hint style="danger" %} If the client or server doesn't support WebSocket, `Application.Update()` has no effect since the connection doesn't exist. Wisej.NET provides alternative client updates through automatic polling. See [Real Time Web Applications](https://docs.wisej.com/docs/concepts/real-time-web-applications) and pollingInterval in [Configuration](https://docs.wisej.com/docs/concepts/configuration). {% endhint %} ## Systems Without WebSocket Older clients or servers may not support WebSocket connections, preventing out-of-bound UI updates from background tasks reaching the client browser. Wisej.NET provides alternative client updates using `Application.StartPolling(milliseconds)` and `Application.EndPolling()` to enable periodic server polling. These methods are ignored when using WebSocket connections. {% tabs %} {% tab title="C#" %} ```csharp // this has no effect when the client and server are connected using WebSocket. Application.StartPolling(1000); Application.StartTask(() => { for (int i = 0; i < 100; i++) { this.label1.Text = "Counting..." + i; Thread.Sleep(1000); } // this has no effect when the client and server are NOT connected using WebSocket. Application.Update(this); // this has no effect when the client and server are connected using WebSocket. Application.EndPolling(); }); ``` {% endtab %} {% tab title="VB.NET" %} ```visual-basic ' this has no effect when the client and server are connected using WebSocket.' Application.StartPolling(1000) Application.StartTask(Sub() For i = 0 To 100 Me.label1.Text = "Counting..." + i Thread.Sleep(1000) Next ' this has no effect when the client and server are NOT connected using WebSocket.' Application.Update(this) ' this has no effect when the client and server are connected using WebSocket.' Application.EndPolling() End Sub) ``` {% endtab %} {% endtabs %} {% hint style="info" %} Check with Microsoft and other technology providers to determine which platforms support WebSocket connections {% endhint %} ## Background Task with Multiple Users Since Background Tasks aren't bound to specific sessions, they can perform global tasks serving multiple users simultaneously. While the previous example showed a single-session task, we can create session-independent tasks. Examples include chat servers, stock market watchers, or any task periodically checking resources. The recommended approach is firing events from the service (background task) and letting user sessions subscribe to these events. Below is a simplified service firing events every second with a subscribing client. {% file src="" %} Download Wisej.BackgroundService.zip {% endfile %} {% tabs %} {% tab title="C#" %} ```csharp static class Program { static Program() { // Start the service. BackgroundService.Start(); } ... } public static class BackgroundService { private static Thread _thread; private static bool _stop = false; public static event BackgroundServiceEventHandler Message; public static void Start() { if (_thread == null) _thread = new Thread(ThreadStart); _stop = false; _thread.Start(); } private static void ThreadStart() { var i = 0; while (!_stop) { Thread.Sleep(1000); i++; if (Message != null) Message(null, new BackgroundServiceEventArgs("Hello #" + i)); } } public static void Stop() { _stop = true; } } ``` {% endtab %} {% tab title="VB.NET" %} ```visual-basic Module Program Sub New() ' Start the service.' BackgroundService.Start() End Sub ... End Mdule Module BackgroundService Private _thread As Thread Private _stop As Boolean Public Event Message As BackgroundServiceEventHandler public Sub Start() If _thread = null Then _thread = new Thread(ThreadStart) End If _stop = false _thread.Start() End Sub Private Sub ThreadStart() Dim i as Int32 While Not _stop Thread.Sleep(1000) i++ If Not Message Is Nothing Then Message(Nothing, New BackgroundServiceEventArgs("Hello #" + i)) End If End While End Sub Public Sub Stop() _stop = true End Sub End Module ``` {% endtab %} {% endtabs %} Handling Events from the Background Service {% tabs %} {% tab title="C#" %} ```csharp private void button1_Click(object sender, EventArgs e) { // Any number of clients can attach to the event from the Background service. if (_subscribed) BackgroundService.Message -= BackgroundService_Message; else BackgroundService.Message += BackgroundService_Message; _subscribed = !_subscribed; this.button1.Text = _subscribed ? "Unsubscribe" : "Subscribe"; } private bool _subscribed; // The event handler is called by the Background service thread, so the thread // doesn't have any knowledge of the session, but "this" refers to this specific // object, which allows Wisej.NET to restore the context while executing the code below. private void BackgroundService_Message(object sender, BackgroundServiceEventArgs e) { // Application.Update() can optionally call a code block in context before updating the UI. Application.Update(this, () => { this.label1.Text = e.Message; }); } ``` {% endtab %} {% tab title="VB.NET" %} ```visual-basic Private Sub button1_Click(sender As Object, e As EventArgs) ' Any number of clients can attach to the event from the Background service.' If _subscribed Then RemoveHander BackgroundService.Message, AddressOf BackgroundService_Message Else AddHandler BackgroundService.Message, AddressOf BackgroundService_Message End If _subscribed = Not _subscribed Me.button1.Text = If (_subscribed, "Unsubscribe", "Subscribe") End Sub Private _subscribed as Boolean ' The event handler is called by the Background service thread, so the thread' ' does not have any knowledge of the session, but "this" refers to this specific' ' object, which allows Wisej.NET to restore the context while executing the code below.' Private Sub BackgroundService_Message(sender As Object, e As BackgroundServiceEventArgs) ' Application.Update() can optionally call a code block in context before updating the UI.' Application.Update(Me, Sub() Me.label1.Text = e.Message End Sub) End Sub ``` {% endtab %} {% endtabs %} The power and simplicity of background tasks in Web applications through the Wisej.NET framework allows handling the browser from the server as if it were a screen directly connected to the server. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.wisej.com/docs/concepts/background-tasks.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. 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