How to initialize and start the .NET MAUI runtime from an iOS app

[rainbowcardiod]

A sample iOS MAUI app created with Visual Studio Code generates an executable binary and many dlls. I believe the binary contains a .NET runtime/interpreter, in fact, using Assembly.LoadFrom() it is possible to load a dll library at runtime. Since on iOS it is not possible to do JIT or dlopen(), I assume the dll of a .NET/MAUI library contains a sort of intermediate byte code. So far, so good.
The question is how to have instead of a binary, a shared library (dylib) containing the runtime/interpreter, so that everything (runtime/interpreter + dll's) can be used by a simple iOS app which knows nothing about .NET. More explicitly, the aim is to:

1. have a library, let's call it libdotnetruntime.dylib
2. include this dylib in an iOS app
3. load the dotnet runtime using this dylib
4. load all "system" dll's of .NET MAUI
5. load the main .dll containing the main code

I've tried many things:

1. cross compiling dotnet with: ./build.sh clr+clr.runtime+libs+packs -os ios -arch arm64 -cross -c Release coreclr-ios and codesigning all .dylib of artifacts/
   1. in the iphone, experimenting with test/.../nativehost run_app or artifacts/bin/testhost/net10.0-ios-Release-arm64/dotnet, but in either cases I get Failed to create CoreCLR, HRESULT: 0x8007FF05
   2. Trying with Hello-iOS app of coreclr-ios. But it fails in coreclr_initialize
2. Then I thought coreclr may not ever work because of JIT. So I turned to experimenting with NativeAOT. With NativeAOT I can create a native library as per examples of runtimelab-NativeAOT. However, I cannot call Assembly.LoadFrom()

Theoretically, something can be done because a sample iOS MAUI App has all it needs to run everything and load dll's. How should it be possible to manually load the MAUI runtime from one or more dylib's and then load the main dll?

[rainbowcardiod]

What is the source code which is responsible to build the main executable binary of an iOS MAUI app? Perhaps, it is little effort to modify it and to let it link a shared library instead of an executable

[vitek-karas]

@rolfbjarne maybe you would know if this is at all possible.

The only supported runtime on iOS is currently mono and NativeAOT. NativeAOT won't work if you need it to load code dynamically.

[rainbowcardiod]

mono would be fine

[rolfbjarne]

This sounds like a duplicate of dotnet/macios#15217.

This is not particularly easy to implement (which is why it hasn't been done), but we're of course happy to help any contributors if someone wants to take on the work.

[rainbowcardiod]

Thank you for your reply, which answers my question. I'd like to ask if there is a hacky way of still doing it. For example, with dotnet publish -r ios-arm64 -f net9.0-ios and find ./obj/ -name main.arm64.mm I found the code of the main executable binary of the sample ios MAUI app. This code does all the lifting of the runtime. Is there a way to print the swiftc or clang compiler flags used to compile it? For example, if it uses clang, by replaying the last stage of compilation and adding -shared the trick would be done. I tried with /bl (binary log) and -v:diag but I cannot find them.

[rolfbjarne]

The binlog has the exact clang commands, I think the main executable is compiled in the _CompileNativeExecutable target. Right-click that target in a binlog viewer, "copy all" (or something like this), paste into a text editor, and you should see the clang commands.

[rainbowcardiod]

Yes, you're right there are all compilation flags in binlog.

It worked!

dotnet publish -f net9.0-ios -r ios-arm64 /bl -c Debug
cd obj/Debug/net9.0-ios/ios-arm64/nativelibraries/
echo _main >> ../mtouch-symbols.list
clang++ -v -shared \
        -framework AuthenticationServices -framework CFNetwork -framework CloudKit -framework Contacts -framework ContactsUI -framework CoreFoundation -framework CoreGraphics -framework CoreImage -framework CoreLocation -framework CoreText -framework Foundation -framework ImageIO -framework Intents -framework LinkPresentation -framework MediaPlayer -framework QuartzCore -framework SafariServices -framework Security -framework UIKit -framework WebKit -framework GSS \
        -lz  -liconv -lcompression -licucore -lobjc \
        -exported_symbols_list ../mtouch-symbols.list  \
        -mios-version-min=15.0 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/  $(ls -1 ../../../../../bin/Debug/net9.0-ios/ios-arm64/*.a | grep -v Microsoft.iOS.registrar.a)   \
         /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/iphoneos/libswiftCompatibility*.a \
        aot-output/arm64/*.o *.o -o DotnetTest.dylib

Then I put DotnetTest.dylib into an Objective C source without main, and linked against DotnetTest.dylib to create a new binary, codesigned both the dylib and the binary, using the entitlements from the obj/ dir, and the app works normally.
Now, I guess more hacking has to be done, but it is a start.

Note: it does not work in Release mode, because I believe it compiles it in aot-only mode.

[steveisok]

The mono runtime has rough library mode support similar to nativeaot by installing the mobile-librarybuilder workload. You'll be able to create a dylib the same way you do in NativeAOT.

In any approach library mode approach, I think you're going to run into the limitation of having to stay within the bounds of C#. Using the mono runtime instead of nativeaot can likely get you past the Assembly.LoadFrom problem.

[rainbowcardiod]

I've tried it now. I followed your README however during compilation I get Using member 'System.Reflection.Assembly.LoadFrom(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types and members the loaded assembly depends on might be removed., so Assembly.LoadFrom does not work, at runtime it says "Failed to load AOT module..... in aot-only mode". What flag should I specify?

[rainbowcardiod]

By looking at mono/msbuild/apple/build/AppleBuild.props, I read "devices should FulltAOT", while for simulators "force interpreter fallback". So I believe it is not supported to use mobile-librarybuilder + Assembly.LoadFrom. Trying to pass /p:MonoForceInterpreter=true does not work either.