using System;

using System.Xml;

using HtmlAgilityPack;

public class Program

{

    public static void Main()

    {

        var html = @"<div><img/></div>";

        HtmlDocument doc = new HtmlDocument();

        doc.LoadHtml(html);

        var src = doc.DocumentNode

                     .SelectSingleNode("//img")

                     .GetAttributeValue("src", "/Assets/Images/Unavailable.png");

        Console.WriteLine(src);

    }

}