Why Does My WebBrowser Control Fail to Fire DocumentCompleted Events When Used in a Separate Thread?

Multi-threaded WebBrowser Control and the DocumentCompleted Event

When processing multiple URLs concurrently, creating a separate WebBrowser control for each URL within its own thread might seem efficient. However, this approach often fails to trigger the DocumentCompleted event reliably. This article explains why and provides a solution.

The core problem lies in the nature of the WebBrowser control. As an ActiveX component, it requires a Single-Threaded Apartment (STA) thread to function correctly. Simply starting a new thread isn't sufficient; the thread needs a message loop to process events. Without this message loop, the DocumentCompleted event, and other crucial events, are never dispatched.

Here's a corrected code example demonstrating the solution:

<code class="language-csharp">private void RunBrowserThread(Uri url) {
    var thread = new Thread(() => {
        var browser = new WebBrowser();
        browser.DocumentCompleted += Browser_DocumentCompleted;
        browser.Navigate(url);
        Application.Run(); // Crucial: Starts the message loop
    });
    thread.SetApartmentState(ApartmentState.STA);
    thread.Start();
}

private void Browser_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e) {
    var browser = (WebBrowser)sender;
    if (browser.Url == e.Url) {
        Console.WriteLine($"Navigated to {e.Url}");
        Application.ExitThread(); // Safely exits the thread
    }
}</code>

The key change is the inclusion of Application.Run() within the thread's execution. This line starts the message pump, enabling the WebBrowser control to receive and process events like DocumentCompleted. The thread then gracefully exits using Application.ExitThread() after the navigation is complete. This ensures proper event handling and prevents unexpected application behavior.

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