How Can I Get C# To Distinguish Between Ambiguous Class Names?

Use aliases:

using HapHtmlDocument = HtmlAgilityPack.HtmlDocument;
using WfHtmlDocument = System.Windows.Forms.HtmlDocument;

You can define an alias for one namespace, e.g:

using hap = HtmlAgilityPack;

and then use the alias instead of the full namespace:

hap.HtmlDocument doc = new hap.HtmlDocument;