[CoreCLR][Fast Deploy] App crashes after an incremental C# edit on device or emulator

[davidnguyen-tech]

Summary

With .NET 11 Preview 6 CoreCLR Debug, a Fast Deploy build/install succeeds after a
comment-only C# edit, but the app crashes on its next launch on both physical devices
and Android emulators.

An app-defined Activity passed to Intent is resolved as java.lang.Object, causing
an ActivityNotFoundException. A clean deployment launches successfully, and
restoring the original source makes the app launch successfully again.

Reproduction

Environment:

• SDK 11.0.100-preview.6.26359.118
• Android workload 37.0.0-preview.6.59
• Physical arm64 Android device or API 36.1 arm64 emulator

Fresh Android and MAUI builds with no runtime or packaging overrides evaluated to the
same CoreCLR Debug/Fast Deploy behavior used below.

Create a fresh app:

dotnet new android -n AndroidFastDeployRepro

Add a second Activity:

[Activity(Label = "Fast Deploy Result")]
public sealed class SecondActivity : Activity
{
}

Launch it from MainActivity.OnCreate:

StartActivity(new Intent(this, typeof(SecondActivity)));

Build and install:

dotnet build AndroidFastDeployRepro.csproj \
  -c Debug -f net11.0-android -t:Install \
  -p:RuntimeIdentifier=android-arm64 \
  -p:UseMonoRuntime=false \
  -p:EmbedAssembliesIntoApk=false \
  -p:AdbTarget="-s DEVICE_SERIAL" \
  /bl:{}

1. Launch the clean deployment. It successfully opens SecondActivity.
2. Append a comment to MainActivity.cs.
3. Run the same build/install command.
4. Launch again.

The incremental launch fails:

android.content.ActivityNotFoundException:
Unable to find explicit activity class
{<package-name>/java.lang.Object}

Restoring the original source and rebuilding makes the launch succeed again.

The same clean-pass / incremental-fail / restored-pass sequence reproduces on a
Samsung A16 and an API 36.1 arm64 Android emulator.

Initial analysis

• The app assembly MVID changes after the comment-only edit.
• _CreateApplicationSharedLibraries relinks libxamarin-app.so with the new typemap.
• _BuildApkFastDev is skipped, so the installed APK retains the old
  libxamarin-app.so.
• Fast Deploy reports success and copies only the updated app DLL/PDB to
  files/.__override__.
• CoreCLR loads the new assembly but performs managed-to-Java lookup with the old
  MVID map.

The clean and incremental APK and libxamarin-app.so hashes are identical, while the
app assembly and typemap MVID differ.

The template repro surfaces as an ActivityNotFoundException. The same mismatch was
also observed in AzureUX as no Java peer type found, followed by SIGABRT from its
unhandled-exception path.

_BuildApkFastDevStaticInputs does not include @(_ApplicationSharedLibrary), even
though _BuildApkFastDev packages that item.

Adding @(_ApplicationSharedLibrary) to the target inputs fixes the failure by
rebuilding and reinstalling the APK whenever the typemap library changes. This keeps
EmbedAssembliesIntoApk=false, but is a correctness fix rather than an assembly-only
Fast Deploy solution.

Related context:

• [CoreCLR] support for debug build typemaps #10065 added the MVID-keyed CoreCLR Debug typemap and noted that Fast Deploy typemap
  support was not implemented.
• [CoreCLR] Support for fastdev assemblies #10113 added CoreCLR Fast Deploy assembly loading, but its device test covered clean
  install/run rather than an incremental app edit.
• [FastDev] Add faster FastDeploy2 #11795 is not involved: this workload predates FastDeploy2 and uses the legacy
  FastDeploy task.

[jonathanpeppers]

@davidnguyen-tech is the problem solved if you set _AndroidFastDevStrategy=FastDeploy?

A new FastDeploy2 was introduced by default:

• [FastDev] Add faster FastDeploy2 #11795

[davidnguyen-tech]

Tried this on the same physical Samsung A16 with _AndroidFastDevStrategy=FastDeploy.

Result was unchanged:

• clean deploy: passes
• incremental GREF logging is broken in multi-process apps #1 after the comment-only edit: fails with the same ActivityNotFoundException for java.lang.Object
• incremental Add Linux sdk items and fix toolchain downloader to really respect HostOS #2 after restoring the source: passes

The incremental binlog confirms _AndroidFastDevStrategy=FastDeploy, the FastDeploy task executed, FastDeploy2 did not execute, and _BuildApkFastDev was skipped.

This workload is 37.0.0-preview.6.59, which predates #11795, so it was already using the legacy FastDeploy implementation.

[jonathanpeppers]

Ok, I think the cause is UseMonoRuntime=false (CoreCLR), .NET 10 works unless I set UseMonoRuntime=false.

[kotlarmilos]

@davidnguyen-tech Could you try running it with this fix #12205?