Web scraping using WebBrowser in a class library

Question

I need to create a method in a class library to get the content of a URL (which may be dynamically populated by JavaScript).

I am clueless, but having googling for the whole day this is what I came up with: (Most of the code is from here)

using System;
using System.Threading.Tasks;
using System.Threading;
using System.Windows.Forms;

public static class WebScraper
{
    [STAThread]
    public async static Task<string> LoadDynamicPage(string url, CancellationToken token)
    {
        using (WebBrowser webBrowser = new WebBrowser())
        {
            // Navigate and await DocumentCompleted
            var tcs = new TaskCompletionSource<bool>();
            WebBrowserDocumentCompletedEventHandler onDocumentComplete = (s, arg) => tcs.TrySetResult(true);

            using (token.Register(() => tcs.TrySetCanceled(), useSynchronizationContext: true))
            {
                webBrowser.DocumentCompleted += onDocumentComplete;
                try
                {
                    webBrowser.Navigate(url);
                    await tcs.Task; // wait for DocumentCompleted
                }
                finally
                {
                    webBrowser.DocumentCompleted -= onDocumentComplete;
                }
            }

            // get the root element
            var documentElement = webBrowser.Document.GetElementsByTagName("html")[0];

            // poll the current HTML for changes asynchronosly
            var html = documentElement.OuterHtml;
            while (true)
            {
                // wait asynchronously, this will throw if cancellation requested
                await Task.Delay(500, token);

                // continue polling if the WebBrowser is still busy
                if (webBrowser.IsBusy)
                    continue;

                var htmlNow = documentElement.OuterHtml;
                if (html == htmlNow)
                    break; // no changes detected, end the poll loop

                html = htmlNow;
            }

            // consider the page fully rendered 
            token.ThrowIfCancellationRequested();
            return html;
        }
    }
}

It currently throws this error

ActiveX control '8856f961-340a-11d0-a96b-00c04fd705a2' cannot be instantiated because the current thread is not in a single-threaded apartment.

Am I close? Is there a fix for the above?

Or if I am off the track, is there a ready solution to get dynamic web content using .NET (that can be called from a class library)?

Answer 1

Here is what I tested in a web application and worked properly.

It uses a WebBrowser control in another thread and returns a Task<string> containing which completes when the browser content load completely:

using System;
using System.Threading.Tasks;
using System.Threading;
using System.Windows.Forms;
public class BrowserBasedWebScraper
{
    public static Task<string> LoadUrl(string url)
    {
        var tcs = new TaskCompletionSource<string>();
        Thread thread = new Thread(() => {
            try {
                Func<string> f = () => {
                    using (WebBrowser browser = new WebBrowser())
                    {
                        browser.ScriptErrorsSuppressed = true;
                        browser.Navigate(url);
                        while (browser.ReadyState != WebBrowserReadyState.Complete)
                        {
                            System.Windows.Forms.Application.DoEvents();
                        }
                        return browser.DocumentText;
                    }
                };
                tcs.SetResult(f());
            }
            catch (Exception e) {
                tcs.SetException(e);
            }
        });
        thread.SetApartmentState(ApartmentState.STA);
        thread.IsBackground = true;
        thread.Start();
        return tcs.Task;
    }
}