.NET 6: `$(AndroidEnablePreloadAssemblies)`=False by default

[jonpryor]

Context: d13d0f9
Context: b90d3ab
Context: PR #5790
Context: PR #5811
Context? 9e6ce03

For .NET 6 we want $(AndroidEnablePreloadAssemblies)=False by default, so that all assemblies don't need to be loaded as part of process startup.

This was done in PR #5790 / d13d0f9 , after which point we found a problem (missed during PR review, alas):

1. .NET 6 +
2. "Debug" build +
3. Fast Deployment +
4. App launch

resulted in a crash (!) in DebuggingTest.ClassLibraryMainLauncherRuns():

android.runtime.JavaProxyThrowable: System.NullReferenceException: Object reference not set to an instance of an object
    at MyLibrary.MainActivity.OnCreate(Bundle bundle)
    at Android.App.Activity.n_OnCreate_Landroid_os_Bundle_(IntPtr jnienv, IntPtr native__this, IntPtr native_savedInstanceState)
    at com.xamarin.classlibrarymainlauncherruns.MainActivity.n_onCreate(Native Method)
    at com.xamarin.classlibrarymainlauncherruns.MainActivity.onCreate(MainActivity.java:29)

The cause of the crash is that, because not all assemblies are loaded as part of process startup, ResourceIdManager.UpdateIdValues() isn't able to update all fields in all Resource types to have the values from the app Resource.designer.cs file. Consequently, the values may be invalid, thus the NullReferenceException.

In order to actually default $(AndroidEnablePreloadAssemblies)=False for .NET 6, we need to figure out how to make this work.

Perhaps we can enable $(AndroidLinkResources)=True (9e6ce03) for all .NET 6 builds? What would the build-time impacts be?

[jonpryor]

I thought I had an idea of a workaround: we should add a handler to the AppDomain.AssemblyLoad event, and fixup resource ids on the assembly when it's loaded.

Then while discussing with @dellis1972 we realized that none of this makes any sense: ResourceIdManager.UpdateIdValues() calls the generated Resource.UpdateIdValues() method, which is e.g.

// From the tests/Xamarin.Forms-Performance-Integration build
namespace Xamarin.Forms.Performance.Integration.Droid {
	public partial class Resource {
		public static void UpdateIdValues()
		{
			global::Xamarin.Forms.Platform.Android.Resource.Animation.abc_fade_in = global::Xamarin.Forms.Performance.Integration.Droid.Resource.Animation.abc_fade_in;
			global::Xamarin.Forms.Platform.Android.Resource.Animation.abc_fade_out = global::Xamarin.Forms.Performance.Integration.Droid.Resource.Animation.abc_fade_out;
			global::Xamarin.Forms.Platform.Android.Resource.Animation.abc_grow_fade_in_from_bottom = global::Xamarin.Forms.Performance.Integration.Droid.Resource.Animation.abc_grow_fade_in_from_bottom;
			// …
		}
	}
}

How does this fail? When $(AndroidEnablePreloadAssemblies) is False, "no" assemblies are preloaded, sure. However, once the above method is invoked, every referenced assembly must be loaded, simply so that the specified field can be resolved and it's value updated.

Thus, why does MainActivity.OnCreate() throw a NullReferenceException? Presumably it's because MainActivity.cs' FindViewById<Button>() invocation is returning null:

https://github.com/xamarin/xamarin-android/blob/af7f7f5da475c5cb8d2d725d77082ba7915bbc7e/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/MainActivity.cs#L26-L30

That still doesn't make sense though; why would button be null? (Is that the actual reason for the NullReferenceException?)

More investigation required.

[jonpryor]

More investigation: https://discord.com/channels/732297728826277939/732297837953679412/836964276438564884

The cause of the issue is that when $(AndroidEnablePreloadAssemblies)=False, the "main" assembly isn't loaded by default. The main assembly is also the assembly which has the Resource.UpdateIdValues() method which is responsible for updating all of the other Resource values (see snippet in previous comment).

Because the main assembly isn't loaded, no assembly is found by ResourceIdManager.UpdateIdValues(), and thus Resource.UpdateIdValues() is never invoked, and thus the resource ids for non-main assemblies are never updated to contain the correct values.

Possible solutions:

1. Always preload the "main" assembly. (The "main" assembly is the assembly created by the "App" .csproj build.)

   @dellis1972 is looking into this now.

2. Otherwise "change" ResourceIdManager.UpdateIdValues() logic so that instead of probing all loaded assemblies for the Resource.UpdateIdValues() method to execute, we specify the type to use "elsewhere", e.g. environment variable, libxamarin-app.so value, etc.

3. ...?

[dellis1972]

Work around #5883