Get links of a html document in order

You can get the correct order by walking the DOM tree using HTML DOM API. The following code does this. Note, I use dynamic to access DOM API. That's because WebBrowser's HtmlElement.FirstChild/HtmlElement.NextSibling don't work for this purpose, as they return null for DOM text nodes.

private void btnGet_Click(object sender, EventArgs e)
{
    Action<object> walkTheDom = null;
    var links = new List<object>();

    // element.FirstChild / NextSibling don't work as they stop at DOM text nodes

    walkTheDom = (element) =>
    {
        dynamic domElement = element;
        if (domElement.tagName == "A")
            links.Add(domElement);
        for (dynamic child = domElement.firstChild; child != null; child = child.nextSibling)
        {
            if (child.nodeType == 1) // Element node?
                walkTheDom(child);
        }
    };

    walkTheDom(this.webBrowser.Document.Body.DomElement);

    string html = links.Aggregate(String.Empty, (a, b) => a + ((dynamic)b).outerHtml + Environment.NewLine);
    MessageBox.Show(html);
}

[UPDATE] If you really need to get a list of HtmlElement objects for <A> tags, instead of dynamic native elements, that's still possible with a little trick using GetElementById:

private void btnGet_Click(object sender, EventArgs e)
{
    // element.FirstChild / NextSibling don't work because they stop on DOM text nodes

    var links = new List<HtmlElement>();
    var document = this.webBrowser.Document;
    dynamic domDocument = document.DomDocument;
    Action<dynamic> walkTheDom = null;

    walkTheDom = (domElement) =>
    {
        if (domElement.tagName == "A")
        {
            // get HtmlElement for the found <A> tag
            string savedId = domElement.id;
            string uniqueId = domDocument.uniqueID;
            domElement.id = uniqueId;
            links.Add(document.GetElementById(uniqueId));
            if (savedId != null)
                domElement.id = savedId;
            else
                domElement.removeAttribute("id");
        }
        for (var child = domElement.firstChild; child != null; child = child.nextSibling)
        {
            if (child.nodeType == 1) // is an Element node?
                walkTheDom(child);
        }
    };

    // walk the DOM for <A> tags
    walkTheDom(domDocument.body);

    // show the found tags
    string combinedHtml = links.Aggregate(String.Empty, (html, element) => html + element.OuterHtml + Environment.NewLine);
    MessageBox.Show(combinedHtml);
}