Get ReadyState from WebBrowser control without DoEvents

Below is a basic WinForms app code, illustrating how to wait for the DocumentCompleted event asynchronously, using async/await. It navigates to multiple pages, one after another. Everything is taking place on the main UI thread.

Instead of calling this.webBrowser.Navigate(url), it might be simulating a form button click, to trigger a POST-style navigation.

The webBrowser.IsBusy async loop logic is optional, its purpose is to account (non-deterministically) for the page's dynamic AJAX code which may take place after window.onload event.

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

namespace WebBrowserApp
{
    public partial class MainForm : Form
    {
        WebBrowser webBrowser;

        public MainForm()
        {
            InitializeComponent();

            // create a WebBrowser
            this.webBrowser = new WebBrowser();
            this.webBrowser.Dock = DockStyle.Fill;
            this.Controls.Add(this.webBrowser);

            this.Load += MainForm_Load;
        }

        // Form Load event handler
        async void MainForm_Load(object sender, EventArgs e)
        {
            // cancel the whole operation in 30 sec
            var cts = new CancellationTokenSource(30000);

            var urls = new String[] { 
                    "http://www.example.com", 
                    "http://www.gnu.org", 
                    "http://www.debian.org" };

            await NavigateInLoopAsync(urls, cts.Token);
        }

        // navigate to each URL in a loop
        async Task NavigateInLoopAsync(string[] urls, CancellationToken ct)
        {
            foreach (var url in urls)
            {
                ct.ThrowIfCancellationRequested();
                var html = await NavigateAsync(ct, () => 
                    this.webBrowser.Navigate(url));
                Debug.Print("url: {0}, html: \n{1}", url, html);
            }
        }

        // asynchronous navigation
        async Task<string> NavigateAsync(CancellationToken ct, Action startNavigation)
        {
            var onloadTcs = new TaskCompletionSource<bool>();
            EventHandler onloadEventHandler = null;

            WebBrowserDocumentCompletedEventHandler documentCompletedHandler = delegate
            {
                // DocumentCompleted may be called several time for the same page,
                // if the page has frames
                if (onloadEventHandler != null)
                    return;

                // so, observe DOM onload event to make sure the document is fully loaded
                onloadEventHandler = (s, e) =>
                    onloadTcs.TrySetResult(true);
                this.webBrowser.Document.Window.AttachEventHandler("onload", onloadEventHandler);
            };

            this.webBrowser.DocumentCompleted += documentCompletedHandler;
            try
            {
                using (ct.Register(() => onloadTcs.TrySetCanceled(), useSynchronizationContext: true))
                {
                    startNavigation();
                    // wait for DOM onload event, throw if cancelled
                    await onloadTcs.Task;
                }
            }
            finally
            {
                this.webBrowser.DocumentCompleted -= documentCompletedHandler;
                if (onloadEventHandler != null)
                    this.webBrowser.Document.Window.DetachEventHandler("onload", onloadEventHandler);
            }

            // the page has fully loaded by now

            // optional: let the page run its dynamic AJAX code,
            // we might add another timeout for this loop
            do { await Task.Delay(500, ct); }
            while (this.webBrowser.IsBusy);

            // return the page's HTML content
            return this.webBrowser.Document.GetElementsByTagName("html")[0].OuterHtml;
        }
    }
}

If you're looking to do something similar from a console app, here is an example of that.

The solution is simple:

    // MAKE SURE ReadyState = Complete
            while (WebBrowser1.ReadyState.ToString() != "Complete") {
                Application.DoEvents();         
            }

// Move on to your sub-sequence code...

Dirty and quick.. I am a VBA guys, this logic has been working forever, just took me days and found none for C# but I just figured this out myself.

Following is my complete function, the objective is to obtain a segment of info from a webpage:

private int maxReloadAttempt = 3;
    private int currentAttempt = 1;

    private string GetCarrier(string webAddress)
    {
        WebBrowser WebBrowser_4MobileCarrier = new WebBrowser();
        string innerHtml;
        string strStartSearchFor = "subtitle block pull-left\">";
        string strEndSearchFor = "<";

        try
        {
            WebBrowser_4MobileCarrier.ScriptErrorsSuppressed = true;
            WebBrowser_4MobileCarrier.Navigate(webAddress); 

            // MAKE SURE ReadyState = Complete
            while (WebBrowser_4MobileCarrier.ReadyState.ToString() != "Complete") {
                Application.DoEvents();         
            }

            // LOAD HTML
            innerHtml = WebBrowser_4MobileCarrier.Document.Body.InnerHtml;  

            // ATTEMPT (x3) TO EXTRACT CARRIER STRING
            while (currentAttempt <=  maxReloadAttempt) {
                if (innerHtml.IndexOf(strStartSearchFor) >= 0)
                {
                    currentAttempt = 1; // Reset attempt counter
                    return Sub_String(innerHtml, strStartSearchFor, strEndSearchFor, "0"); // Method: "Sub_String" is my custom function
                }
                else
                {
                    currentAttempt += 1;    // Increment attempt counter
                    GetCarrier(webAddress); // Recursive method call
                } // End if
            } // End while
        }   // End Try

        catch //(Exception ex)
        {
        }
        return "Unavailable";
    }