The future of Microsoft MAUI (and Xamarin Forms)

Oliver Brown
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Since Google seems to like my post about the future of Xamarin Forms so much (and I have a slight history of such posts), I’d figure I’d post an update about interesting things happening in the Xamarin Forms repo specifically related to MAUI.


The change that actually made me write this post - a large PR with 5000+ changed files that changes the Xamarin Forms name to MAUI.

Not much of a thing for actual functionality, but a significant symbolic milestone.

Handlers and the great big architecture shift

.NET MAUI will completely change the way renderers are handled in Xamarin Forms. There are many advantages of doing it the new way, but the mechanics of how it is done are fairly complex. This video by Javier Suárez covers it well.

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This is all happening right now in the main-handler branch.


While I was writing this, work officially moved to the dotnet/maui repo and it is accepting pull requests directly.

AppHost and Microsoft.Extensions

Originally an ASP.NET concept, that then migrated its way to Windows client development, this provides a common way to provide dependency injection, logging, and lots of other infrastructure stuff. In isolation, the pattern and implementation is good and will make it easier to override certain things in MAUI (such as handlers). It’s also useful in a wider sense since it will make configuring different styles of .NET apps more similar.

Single project

Over the past couple of years there has been a move towards producing Xamarin libraries (and .NET libraries in general) using a single multi-targeted project. The most significant is probably Xamarin Essentials. This PR adds support for creating applications following the same pattern.

Merging in Xamarin Essentials

There is a lot of functionality in Xamarin Essentials that Xamarin Forms would like to use. Likewise there is some functionality in Forms that is useful when not using Forms. This lead to some overlap in functionality (and occasionally overlap in APIs but not a perfect match in functionality).

There was an attempt to add Essentials as a dependency of Forms but it faced some problems, and there was a “change of plans”.

Now the solution is to have Forms and Essentials in the same repo. I hope Essentials remains available as its own Nuget package (and it looks like that will be the case).


Resizertizer.NT, like its predecessor Resizetizer, is a package for generating platform specific images in all the right sizes at build time.

Managing image assets across iOS and Android (and using Visual Studio) has always been an unpleasant process. This tool makes it much easier and will be included in MAUI by default.

The future of Xamarin Forms

Oliver Brown
— This upcoming video may not be available to view yet.

Microsoft have just announced the future of Xamarin and Xamarin forms - the .NET Multi-platform App UI (or MAUI for short). As a name, it’s not great. The highlights are as follows:

  • It’s an evolution of Xamarin Forms. It basically is Xamarin Forms, but finally accepting some breaking changes. To be honest, I’m hoping for a lot since there is a lot of weirdness in Xamarin Forms that has been holding it back.
  • Single project, multi-targeted. It took a long time to get to the point where this was possible. From shared projects, to PCL projects, through .NET Standard. This should make things a lot easier.
  • Still based on platform renderers using native controls. This is a mixed bag. Using native controls has long been a selling point of Xamarin (with or without Forms). With the rise of Flutter this has been shown to be less important. Many people have been asking for consistent platform agnostic renderers instead.
  • The end of “Xamarin” as a name. Some time in the .NET 6 timeline (end of 2021) Xamarin.iOS will become .NET for iOS and Xamarin.Android will be .NET for Android. I have mixed feeling about this since this was a fairly succinct way to describe by top skillset.

I also already have my own enhancement issue submitted.

Accepting money on the iOS App Store - acquiring an EIN

Oliver Brown
— This upcoming video may not be available to view yet.

To sell an app (or an in-app purchase) on the iOS App Store as a non-US citizen, a tax ID is required* to fill in a W8 form. That is either an ITIN (Individual Taxpayer Identification Number) or EIN (Employer Identification number).

As an individual, the ITIN would appear the most correct option. The last time I investigated it, it was a slow process that would require notarised copies of supporting documentation.

Getting an EIN is much easier, and despite the name, does not require you to have employees. All you need to do is be in business (which selling in the app store is) and being a sole proprietor (sole trader in the UK) counts.

In theory, you fill in form SS-4 to get one. It’s very straightforward, and even has an explanation of exactly which parts you need to fill in (look for notes for “IF the applicant… Is a foreign person needing an EIN to comply with IRS withholding regulations”.

In practice, it is even easier than that. You can apply for one over the phone. They will basically ask you the same questions as on the form, and then give you the EIN there and then. I called “first thing in the morning” (6am for them, 11am for me) and got an answer immediately (they warn you that you could be waiting an hour during busier times). The whole process took less than 10 minutes. The hardest part was confirming how my address should be written down. The net result of all this is that Tic-tac-toe Collection’s single in-app purchase has been enabled (at least for beta testers).

* Whether it is actually required is unclear. The old Xbox 360 program, Xbox Live Indie Games, allowed developers to sell without a tax ID, but a 30% withholding tax would be deducted from your earnings. I believe the best result would be the same happening here. It’s also possible Apple would just not allow you to process sales at all.

ListView improvements in Xamarin Forms

Oliver Brown
— This upcoming video may not be available to view yet.

Since becoming open source, it has become possible to find out potential upcoming features in Xamarin Forms by just poking around the active branches in the repository (macOS support was visible in the repo before any announcement). One of them is lv2spike. From just reading the commit messages, it seems this is a new CollectionView, based on UICollectionView for iOS and RecyclerView for Android. This is something that has been needed for a while, but is a big enough undertaking that I understand why it has taken a while. After all the branch suggests this is still just a spike. There are quite a lot of feature requests for the Xamarin Forms ListView that are just not possible (like this one for horizontal layout) mainly because the iOS implementation is based on UITableView. This will open lots of possibilities. My biggest concern is that despite the push forward with features, Xamarin Forms is accruing bugs even faster, and with the expanded platform support this could just get worse…

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[SOLVED] System.ExecutionEngineException: Attempting to JIT compile method

Oliver Brown
— This upcoming video may not be available to view yet.

TLDR: Check multiple references to the same nuget package are all on the same version if you use the Mono linker.

Since my ability to post regularly on things I’m interested in is not great, I figured I could at least post stuff that might be useful. I recently upgraded a Xamarin iOS app from the “classic” (32bit only) API to the Unified API. After doing so I got the error message:

System.ExecutionEngineException: Attempting to JIT compile method

This is caused by the Xamarin (Mono) linker removing code that is only referenced dynamically. The usual solution is to let the compiler know somehow that you are using the code (using a Preserve attribute if it's your own code or something like MvvmCross's [LinkerPleaseInclude.cs]( otherwisr). In my case, this did not fix the problem. It turns out the Unified API upgrade was a red herring. I had also updated a few nuget packages at the same time. One of them was used in several projects, but I'd missed updating one of them (so I had Project A using v1 of a package and Project B using v2 of a package). This meant my efforts to stop the linker from removing some stuff only worked on one version of the package.
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