In Windows Phone, use HtmlAgilityPack to add a link to an image.

I want to add a link to every image in the HTML with the value of the "href" attribute be the "src" attribute of the image. Namely changing the

 "<p> <img src="test.jpg"/></p>"

to

<p><a href="test.jpg"><img src="jpg"/></a></p>

And my code coming:

using HtmlAgilityPack;
var imgs = document.DocumentNode.Descendants("img");
foreach (HtmlNode node in imgs)
{
    if (node.ParentNode.Name != "a")
    {
        string replaceStr = string.Format("<a href=\"{0}\">{1}</a>", node.GetAttributeValue("src", null), node.OuterHtml);
        //node.OuterHtml= replaceStr;   It doesn't work, the outerHtml is readonly
        //node.ParentNode.RemoveChild(node, true);
    }
}

So how should I modify my code to make it work?

Updated: after updating my code:

var imgs = document.DocumentNode.Descendants("img");
            foreach (var node in imgs)
            {
                if (node.ParentNode.Name != "a")
                {
                    var a = document.CreateElement("a");
                    a.SetAttributeValue("href", node.GetAttributeValue("src", null));
                    a.ChildNodes.Add(node);
                    node.ParentNode.ReplaceChild(a, node);
                }
            }

An error shows up "Unhandled InvalidOperationException ".