Converting .ashx handlers to .NET 5/6

I've been trying to figure out how to convert my site to .NET 5 or 6 for a long while. The pages are written in PHP (because that was the only thing available to me on my web host at the time) but there are many .ashx handlers that were added when I originally decided to move to ASP.NET but never got round to completing the job. But the .NET Framework isn't going to be improved or added to in the future so moving to .NET 5/6 needs to happen at some point. It's not a simple process, the differences are fairly major, but I think I've finally figured out a path to making the move.

My main concern was that I wanted to have a common code base so I can reuse code and continue to work on my current site whilst I set up the updated one, rather than rewriting the whole thing from scratch. A rewrite would certainly be simpler but the switch over would almost certainly be a disaster.

The first step was to work on assemblies used by the site. The Portability Analyzer helped here in figuring out how portable my code was (although it's not generally too helpful in explaining what to do with APIs that are not available in .NET 5). Fortunately it wasn't too difficult to move .NET Coords to .NET Standard 2, meaning it could be used from old and new .NET. I used the .NET Upgrade Assistant to convert to the latest SDK project format. Luckily there were cross platform versions of the third party assemblies I use.

One other thing I did was rip out Entity Framework, which I've never been able to get along with and move from the Oracle MySql ADO.NET connector to MySqlConnector. Like everything from Oracle on Windows, their MySql connector is a complete dumpster fire.

Once that was done, I decided the next step was to move all my .ashx handler code into a new assembly. So every .ashx file now looks something like

<%@ WebHandler Language="C#" Class="DoogalCode.AdministrativeAreasCSV" %>

I configured that assembly to target .NET 4.8 and .NET 5, since there would have to be differences between them. At this point, the .NET 4.8 assembly compiled fine (although with many warnings about using things from System.Web even though that's perfectly fine in .NET 4.8!) and the .NET 5 assembly failed with a gazillion errors. 

The next step was to build the .NET 5 assembly without errors. I decided the simplest way to do that was to build .NET 5 only versions of all the things the compiler was complaining about. This meant things like HttpContext, HttpRequest, HttpResponse and IHttpHandler. The properties and methods didn't do anything except throw an exception, I didn't want the code to run, just compile. I'd figure out the details of  how to implement those methods as I went through each handler and got them working in .NET 5.

Once I'd done that, I needed a .NET web site. Since my front end is written in PHP, I decided to take a look at PeachPie, which claims to be a PHP compiler for .NET. And I am mightily impressed. I created a new PeachPie project, copied my PHP files across and it just worked. I had my website running in .NET 5 in a few minutes. Admittedly I don't do anything very complicated in my PHP, so I can't be sure it's perfect but it's certainly perfect for my needs.

But I still had a bunch of .ashx handlers to get working in .NET 5. I added a method to my website startup class that let me use my dummy IHttpHandler and HttpContext classes to call into my handlers

private static void MapHandler<THandler>(IApplicationBuilder app, string path) where THandler : IHttpHandler, new()

{

  app.Map(path, (app2) =>

  {

    app2.Run(async context =>

    {

      var handler = new THandler();

      await Task.Run(() =>

      {

        handler.ProcessRequest(new DoogalCode.HttpContext(context));

      });

    });

  });

}

Then for each handler I mapped URL paths to the handler via this method, like so

MapHandler<CountiesCSV>(app, "/CountiesCSV");

Now I could call my handlers and start to fix up the methods in my dummy classes. Which is where I am currently, going through each handler fixing issues as I find them, whilst still being able to work on my live website. Once that is done and has gone live, I may try to figure out how to finally move my PHP code to proper .NET

Addendum - This all worked out really well. I was able to retarget all my classic .NET code to .NET 5, whilst continuing to keep the site running. Switching over to the new website was relatively painless, with little downtime. Moving to .NET 6 was straightforward. I was then able to finally start replacing PHP pages with .NET Razor pages, which I'm currently working through. I've also recently learned that Microsoft are developing their own adapters for System.Web.HttpContext etc to make the transition easier, which I wish I'd known sooner!