using System;

using System.Text;

using HtmlAgilityPack;

using System.IO;

using System.Xml;

public class Program

{

    public static void Main()

    {

        var doc = new HtmlDocument();                    

        doc.LoadHtml(@"<div class=""buy-container"">

<form action=""/content/action"" method=""post"">

<button class=""product-buy"">buy</button>

<input type=""hidden"" name=""ActionAddToBasket"">

<input type=""hidden"" value=""198300"" name=""ContentNodeID"">

<input type=""hidden"" value=""778768"" name=""ContentObjectID"">

<input type=""hidden"" value=""full"" name=""ViewMode"">

</form>

</div>");

        var val = doc

                      .DocumentNode

                      .SelectSingleNode("//input[@type='hidden' and @name='ContentObjectID']")

                      .Attributes["value"].Value;

        val.Dump();

    }

}