In order to use MessagePack on the iOS or in general with AOT, you need to use mpc.exe to generate a resolver before building your main projects. Since Xamarin and ASP.NET are all about code sharing, usually a good practice is to create a shared project or a library containing model definitions, and referencing it in both server and client projects.
This raises the question as where to embed the script to run mpc.exe in order to facilitate the automatic generation and updating the resolver class.
One problem is that mpc.exe does not work with Shared Projects (.shproj), and as of now only accepts .csproj files as the input. So one thing to do is to create a stub/dummy project containing only the model classes, and feeding it to mpc.exe.
The frustration comes when mpc.exe doesn’t like an input. In such cases, it merely generates a blank useless resolver class (e.g. MessagePackGenerated.cs) without giving any errors. You have to manually investigate everything in order to find out what goes wrong.
One of the main cases in which I realized that mpc.exe doesn’t work properly, is when you don’t have the correct version of MessagePack NuGet installed on the stub project. Make sure you install MessagePack in that project.