how to convert Htmlnode of HtmlAgilityPack to webbrowser HtmlElement

Thank you everyone. After thinking and programming for a almost whole day, I came to decision that i have to use native htmlElement instead of htmlagilitypack HtmlNode, because I want to input text into Htmlelement in webbrowser . so here is the code that i came up with. still i would appreciate if someone shows a solution with htmlagilitypack.

    public HtmlElement selectHtmlNode(string xPath, HtmlElement htmlElement)
    {
        string currentNode;
        int indexOfElement;

        //get string representation of current Tag.
        if (xPath.Substring(1,xPath.Length-2).Contains('/'))
            currentNode = xPath.Substring(1, xPath.IndexOf('/', 1) - 1);
        else
            currentNode = xPath.Substring(1, xPath.Length-1);
        //gets the depth of current xPath
        int numOfOccurence = Regex.Matches(xPath, "/").Count;

        //gets the children's index
        int.TryParse(Regex.Match(currentNode, @"\d+").Value, out indexOfElement);

        //if i have to select nth-child ex: /tr[4]
        if (indexOfElement > 1)
        {
            currentNode = currentNode.Substring(0, xPath.IndexOf('[') - 1);
            //the tag that i want to get
            if (numOfOccurence == 1 || numOfOccurence == 0)
            {
                return htmlElement.Children[indexOfElement - 1];
            }
            //still has some children tags
            if (numOfOccurence > 1)
            {
                int i = 1;
                //select nth-child
                foreach (HtmlElement tempElement in htmlElement.Children)
                {
                    if (tempElement.TagName.ToLower() == currentNode && i == indexOfElement)
                    {
                        return selectHtmlNode(xPath.Substring(xPath.IndexOf('/', 1)), tempElement);
                    }
                    else if (tempElement.TagName.ToLower() == currentNode && i < indexOfElement)
                    {
                        i++;
                    }
                }
            }
        }
        else
        {
            if (numOfOccurence == 1 || numOfOccurence == 0)
            {
                return htmlElement.FirstChild;
            }
            if (numOfOccurence > 1)
            {
                foreach (HtmlElement tempElement in htmlElement.Children)
                {
                    if (tempElement.TagName.ToLower() == currentNode)
                    {
                        return selectHtmlNode(xPath.Substring(xPath.IndexOf('/', 1)), tempElement);
                    }
                }
            }
        }
        return null;
    }

function is called in this way. where htmlController is instance of some class.

HtmlElement currentElement = htmlController.selectHtmlNode("/body/form/div/div[2]/div/div/input", webBrowser.Document.GetElementsByTagName("html")[0]);
currentElement.SetAttribute("Value", "hello world");