Porting an existing C# dll to be a full WinRT Metro assembly is not trivial. I recently gave a presentation on how to do this.
Here are the basic steps
- Take the existing experimental Win8 Metro C# project in the source control here on codeplex. Under the solutions folder you'll find the HtmlAgilityPack.Metro.sln file
- Next is to go through every public class and change it to sealed
- Then go through and make sure no fields are exposed publicly
- The ugly part is at hand, compile and start fixing any other compilation errors
I currently have not attempted to do this with HAP for a few reasons. 1 I know it may be a long process to get it all working and 2 it will probably introduce breaking changes for the other versions of HAP.