android-slim-bindings

name

android-slim-bindings

description

Create and update slim/native platform interop bindings for Android in .NET MAUI and .NET for Android projects. Guides through creating Java/Kotlin wrappers, configuring Gradle projects, resolving Maven dependencies, generating C# bindings, and integrating native Android libraries using the Native Library Interop (NLI) approach. Use when asked about Android bindings, AAR/JAR integration, Kotlin interop, Maven dependencies, or bridging native Android SDKs to .NET.

When to use this skill

Activate this skill when the user asks:

• How do I create Android bindings for a native library?
• How do I wrap an Android SDK for use in .NET MAUI?
• How do I create slim bindings for Android?
• How do I use Native Library Interop for Android?
• How do I bind a Kotlin library to .NET?
• How do I bind a Java library to .NET?
• How do I integrate an AAR or JAR into .NET MAUI?
• How do I create a Java/Kotlin wrapper for a native Android library?
• How do I update Android bindings when the native SDK changes?
• How do I fix Android binding build errors?
• How do I expose native Android APIs to C#?
• How do I resolve Maven dependencies for Android bindings?
• How do I handle AndroidX dependencies in bindings?
• How do I fix "Java dependency is not satisfied" errors?
• How do I use AndroidMavenLibrary in my binding project?

Overview

This skill guides the creation of Native Library Interop (Slim Bindings) for Android. This modern approach creates a thin native Java/Kotlin wrapper exposing only the APIs you need from a native Android library, making bindings easier to create and maintain.

When to Use Slim Bindings vs Traditional Bindings

Scenario	Recommended Approach
Need only a subset of library functionality	Slim Bindings
Easier maintenance when SDK updates	Slim Bindings
Prefer working in Java/Kotlin for wrapper	Slim Bindings
Better isolation from breaking changes	Slim Bindings
Complex libraries with many dependencies	Slim Bindings
Need entire library API surface	Traditional Bindings
Creating bindings for third-party developers	Traditional Bindings
Already maintaining traditional bindings	Traditional Bindings

Inputs

Parameter	Required	Example	Notes
libraryName	yes	FirebaseMessaging, OkHttp	Name of the native Android library to bind
bindingProjectName	yes	MyBinding.Android	Name for the C# binding project
dependencySource	no	maven, aar, jar	How the native library is distributed
targetFrameworks	no	net9.0-android	Target frameworks (default: latest .NET Android)
exposedApis	no	List of specific APIs	Which native APIs to expose (helps scope the wrapper)
mavenCoordinates	no	com.example:library:1.0.0	Maven coordinates if library is from Maven repository

Project Structure

MyBinding/
 android/
 native/                              # Android Studio/Gradle project   
 app/      
 src/main/         
 java/com/example/mybinding/            
 DotnetMyBinding.java  # Java wrapper implementation                
 kotlin/com/example/mybinding/            
 DotnetMyBinding.kt    # Or Kotlin wrapper                
 build.gradle.kts         
 settings.gradle.kts      
 build.gradle.kts      
 MyBinding.Android.Binding/   
 MyBinding.Android.Binding.csproj       
 Transforms/       
 Metadata.xml           
 sample/
 MauiSample/                          # Sample MAUI app   
 MauiSample.csproj       
 MainPage.xaml.cs       
 README.md

Step-by-step Process

For detailed code examples and full implementation walkthroughs, see references/android-slim-bindings-implementation.md.

Step 1: Analyze Dependencies First

Before creating any bindings, analyze the complete dependency tree of your target library. This is the most critical step and where most binding projects fail.

Prerequisites: JDK 17+, Android SDK with ANDROID_HOME set, .NET SDK 9+. You don't need Gradle installed globally; the Gradle Wrapper downloads the correct version automatically.

Step 2: Create the Android Library Project

Choose one of these approaches:

• Option A: Android Studio (recommended for GUI-based projects: create a "No Activity" project, then add an Android Library module)
• Option B: gradle init with Wrapper (recommended for CLI-based projects: scaffold with Gradle using the Wrapper, then convert to an Android library)
• Option C: CommunityToolkit NativeLibraryInterop template (clone and use the CommunityToolkit NativeLibraryInterop template as your starting point)
• Option D: Manual creation with Gradle Wrapper (create the project structure manually using the Gradle Wrapper)

After project creation, configure settings.gradle.kts, root build.gradle.kts, and app/build.gradle.kts with your library dependency.

Step 3: Analyze the Full Dependency Tree

./gradlew app:dependencies --configuration releaseRuntimeClasspath

resolution strategy.

Understanding the output: -> = version conflict resolution, (*) = deduplication, (c) = constraint.

Step 4: Create the Wrapper Class

Create a Java or Kotlin wrapper in app/src/main/java/ (or kotlin/) that exposes only the APIs you need. Key guidelines:

• Use simple, marshallable types (String, boolean, int, byte[], Context, View)
• Use callback interfaces for async operations (not Kotlin coroutines or lambdas)
• Prefix class names with Dotnet for clarity
• Add @JvmStatic on all Kotlin methods for static access from C#
• Use object in Kotlin for singleton pattern

Step 5: Build the AAR

./gradlew :app:assembleRelease
# Output: app/build/outputs/aar/app-release.aar

Step 6: Create the C# Binding Project

dotnet new androidbinding -n MyBinding.Android.Binding

Key .csproj elements:

• < reference the built AARAndroidLibrary>
• < NuGet packages for transitive dependenciesPackageReference>
• <AndroidMavenLibrary Bind=" Maven deps without NuGet equivalentfalse">
• < compile-time only dependenciesAndroidIgnoredJavaDependency>

Configure Transforms/Metadata.xml for namespace renaming and parameter names.

Step 7: Resolve Dependencies

Build and resolve XA4241/XA4242 errors using this decision tree:

For each unsatisfied dependency:

 Is there a XA4242 suggestion?
 Install the suggested NuGet package

 Is the dependency needed from C#?
 Find/create a binding NuGet or create your own binding
 Use AndroidMavenLibrary with Bind="false"

 Is it a compile-time only dependency (annotations, processors)?
 Use AndroidIgnoredJavaDependency

 Create a separate binding project or include AAR/JAR directly

Common Maven-to-NuGet Mappings

Maven Artifact	NuGet Package
androidx.annotation:annotation	Xamarin.AndroidX.Annotation
androidx.core:core	Xamarin.AndroidX.Core
androidx.core:core-ktx	Xamarin.AndroidX.Core.Core.Ktx
androidx.appcompat:appcompat	Xamarin.AndroidX.AppCompat
androidx.fragment:fragment	Xamarin.AndroidX.Fragment
androidx.activity:activity	Xamarin.AndroidX.Activity
androidx.recyclerview:recyclerview	Xamarin.AndroidX.RecyclerView
androidx.lifecycle:lifecycle-common	Xamarin.AndroidX.Lifecycle.Common
org.jetbrains.kotlin:kotlin-stdlib	Xamarin.Kotlin.StdLib
org.jetbrains.kotlinx:kotlinx-coroutines-core	Xamarin.KotlinX.Coroutines.Core
com.google.android.material:material	Xamarin.Google.Android.Material
com.squareup.okhttp3:okhttp	Square.OkHttp3
com.google.code.gson:gson	GoogleGson

Finding NuGet Packages

dotnet package search "artifact=androidx.core:core" --source https://api.nuget.org/v3/index.json

Step 8: Customize Bindings with Metadata

Edit Transforms/Metadata.xml to rename packages, classes, parameters, and remove internal types. Examine obj/Debug/net9.0-android/api.xml after building to construct correct XPath expressions.

Step 9: Build and Verify

dotnet build -c Release

Step 10: Use in Your MAUI App

Add a conditional <ProjectReference>, initialize in MauiProgram.cs using ConfigureLifecycleEvents, and implement callback interfaces by extending Java.Lang.Object.

Updating Bindings When Native SDK Changes

1. Update native dependency version in app/build.gradle.kts
2. Re-analyze dependencies with ./gradlew app:dependencies
3. Update the Java/Kotlin wrapper if APIs changed
4. Rebuild the AAR with ./gradlew :app:assembleRelease
5. Update NuGet packages and Maven dependencies in binding .csproj
6. Rebuild C# binding and fix any new XA4241/XA4242 errors
7. Update Metadata.xml for new classes/methods
8. Test with sample app

For detailed update instructions, see references/android-slim-bindings-implementation.md.

Troubleshooting

Error	Cause	Solution
XA4241 / XA4242	Missing transitive dependency	Use NuGet, AndroidMavenLibrary Bind="false", or AndroidIgnoredJavaDependency
"Type is defined multiple times"	Duplicate dependency	Check NuGet/AAR overlap, use Bind="false"
"Class does not implement interface member"	Covariant return types	Add custom implementation in Additions/ folder
"Cannot find symbol"	Missing Gradle dependency	Verify build.gradle.kts dependencies, rebuild AAR
NoClassDefFoundError (runtime)	Dependency ignored incorrectly	Remove from AndroidIgnoredJavaDependency, satisfy properly
UnsatisfiedLinkError (runtime)	Missing .so native library	Include via <AndroidNativeLibrary>, check ABI compatibility
Callback not invoked	Wrong thread / GC'd reference	Use MainThread.BeginInvokeOnMainThread(), hold strong reference

For detailed troubleshooting with code examples, see references/android-slim-bindings-implementation.md.

Quick Reference

Type Mapping

Java/Kotlin Type	C# Type	Notes
String	string	Direct mapping
boolean / Boolean	bool	Direct mapping
int / Integer	int	Direct mapping
long / Long	long	Direct mapping
double / Double	double	Direct mapping
float / Float	float	Direct mapping
byte[]	byte[]	Direct mapping
Context	Android.Content.Context	Pass from platform
View	Android.Views.View	For view creation
JSONObject	Org.Json.JSONObject	For complex data
Callback interface	IJavaObject implementation	See callback pattern

Things to Avoid in Wrapper API

Avoid	Use Instead
Kotlin coroutines	Callback interfaces
Kotlin lambdas	Java-style interfaces
Kotlin data classes	Simple classes with getters
Kotlin sealed classes	Enums or class hierarchy
Generics (complex)	Specific types or Object
Rx/Flow observables	Callback interfaces

MSBuild Items Reference

Item	Purpose
<AndroidLibrary>	Include AAR/JAR in binding
<AndroidMavenLibrary>	Download and include from Maven
<AndroidIgnoredJavaDependency>	Ignore a dependency (compile-time only)
<AndroidNativeLibrary>	Include .so files
<TransformFile>	Metadata transform files
<PackageReference>	NuGet package reference

Common Attributes

Attribute	Values	Purpose
Bind	true/false	Whether to generate C# bindings
Pack	true/false	Whether to include in NuGet package
JavaArtifact	groupId:artifactId	Declare what Maven artifact this satisfies
JavaVersion	version	Version of the Maven artifact
Repository	Central/Google/URL	Maven repository to download from

Resources

Official Documentation

• Binding a Java Library (Microsoft Docs)
• Binding from Maven (Microsoft Docs)
• Java Dependency Verification
• Resolving Java Dependencies
• Java Bindings Metadata
• Native Library Interop - .NET Community Toolkit

Version Discovery

• Android Gradle Plugin Releases
• Gradle Releases
• Kotlin Releases

Templates and Examples

• Maui.NativeLibraryInterop Repository
• Xamarin.AndroidX Repository

Reference Files

• android-bindings-guide.md — comprehensive binding reference
• android-slim-bindings-implementation.md — detailed code examples, troubleshooting, and complete scripts
• For iOS bindings, use the ios-slim-bindings skill

Output Format

When assisting with Android slim bindings, provide:

1. Dependency analysis - Full dependency tree and NuGet mapping strategy
2. Project structure - File/folder layout for the binding
3. Wrapper code - Complete Java or Kotlin implementation
4. Gradle configuration - build.gradle.kts with dependencies
5. C# binding project - .csproj with proper dependency references
6. Metadata transforms - Metadata.xml for namespace and parameter renaming
7. Usage examples - How to call the binding from MAUI/C#
8. Troubleshooting guidance - Common issues and solutions for the specific library

Always verify:

• All transitive dependencies are accounted for
• NuGet packages match required Maven versions
• Wrapper methods use simple, marshallable types
• Callback interfaces are properly designed
• Metadata.xml renames parameters to meaningful names