android-slim-bindings

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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

The recommended project structure for Native Library Interop:

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

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

Ensure you have:

  • JDK 17+

  • Android SDK with ANDROID_HOME environment variable set

  • .NET SDK 9+ (recommended for Java Dependency Verification)

Note: You don't need Gradle installed globally—we'll use the Gradle Wrapper which downloads the correct version automatically.

Step 2: Create the Android Library Project

There are multiple approaches to create the native Android wrapper project. Choose the one that best fits your workflow.

Option A: Use Android Studio (Recommended for GUI)

The most reliable way to create a properly configured Android library:

  • Open Android Studio

  • File → New → New Project

  • Select "No Activity" template

  • Configure:

  • Name: MyBindingNative

  • Package name: com.example.mybinding

  • Language: Java or Kotlin

  • Minimum SDK: API 21

  • File → New → New Module

  • Select "Android Library"

  • Name it app (or your preferred module name)

This creates a properly structured project with the latest Gradle plugin versions.

Option B: Use gradle init with Wrapper (CLI)

Use Gradle's built-in initialization to scaffold a project, then convert it to Android:

Set your binding name

BINDING_NAME="MyBinding" PACKAGE_NAME="com.example.mybinding"

Create directory structure

mkdir -p ${BINDING_NAME}/android/native mkdir -p ${BINDING_NAME}/android/${BINDING_NAME}.Android.Binding/Transforms mkdir -p ${BINDING_NAME}/sample/MauiSample

cd ${BINDING_NAME}/android/native

Download and use Gradle wrapper (no global Gradle install needed)

This downloads gradle-wrapper.jar and creates gradlew scripts

curl -sL https://services.gradle.org/distributions/gradle-9.2.1-bin.zip -o gradle.zip unzip -q gradle.zip ./gradle-9.2.1/bin/gradle wrapper --gradle-version 9.2.1 rm -rf gradle.zip gradle-9.2.1

Initialize a basic Kotlin library project

./gradlew init --type kotlin-library --dsl kotlin --project-name ${BINDING_NAME}Native --package ${PACKAGE_NAME} --no-split-project --java-version 17

The project needs to be converted to Android Library (see next section)

Note: gradle init creates a standard JVM library, not an Android library. You'll need to modify the generated files to add Android support (see "Converting to Android Library" below).

Option C: Clone from Template Repository

Use the Community Toolkit template as a starting point:

BINDING_NAME="MyBinding"

Clone the template

git clone https://github.com/AdrianSimionescu/NativeLibraryInterop-Template.git ${BINDING_NAME} cd ${BINDING_NAME}

Or use the official CommunityToolkit repo structure

git clone --depth 1 https://github.com/AdrianSimionescu/NativeLibraryInterop-Template.git ${BINDING_NAME}

Rename files and references

find . -type f -name ".gradle" -exec sed -i '' 's/TemplateBinding/'"${BINDING_NAME}"'/g' {} ;

Option D: Manual Creation with Gradle Wrapper

For full control, create the project structure manually but use proper Gradle wrapper initialization:

BINDING_NAME="MyBinding" PACKAGE_NAME="com.example.mybinding" PACKAGE_PATH="${PACKAGE_NAME//.//}"

Create directory structure

mkdir -p ${BINDING_NAME}/android/native/app/src/main/java/${PACKAGE_PATH} mkdir -p ${BINDING_NAME}/android/${BINDING_NAME}.Android.Binding/Transforms mkdir -p ${BINDING_NAME}/sample/MauiSample

cd ${BINDING_NAME}/android/native

Create Gradle wrapper (downloads correct Gradle version automatically)

Using gradle wrapper from a temporary init

mkdir -p tmp && cd tmp curl -sL https://services.gradle.org/distributions/gradle-9.2.1-bin.zip -o gradle.zip unzip -q gradle.zip && ./gradle-9.2.1/bin/gradle wrapper --gradle-version 9.2.1 mv gradlew gradlew.bat gradle ../ && cd .. && rm -rf tmp

Now create the project files (see below)

Creating Gradle Build Files

After setting up the wrapper, create the build files. These files define the project structure and can be version-controlled:

settings.gradle.kts:

pluginManagement { repositories { google() mavenCentral() gradlePluginPortal() } }

dependencyResolutionManagement { repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS) repositories { google() mavenCentral() // Add custom repositories as needed // maven { url = uri("https://jitpack.io") } } }

rootProject.name = "MyBindingNative" include(":app")

build.gradle.kts (root):

plugins { // Use version catalog or explicit versions // Check latest versions at: https://developer.android.com/build/releases/gradle-plugin id("com.android.library") version "8.2.2" apply false id("org.jetbrains.kotlin.android") version "1.9.22" apply false }

app/build.gradle.kts:

plugins { id("com.android.library") // Include kotlin plugin only if using Kotlin // id("org.jetbrains.kotlin.android") }

android { namespace = "com.example.mybinding"

// Check latest SDK versions: https://developer.android.com/about/versions
compileSdk = 34

defaultConfig {
    minSdk = 21
    consumerProguardFiles("consumer-rules.pro")
}

buildTypes {
    release {
        isMinifyEnabled = false
        proguardFiles(
            getDefaultProguardFile("proguard-android-optimize.txt"),
            "proguard-rules.pro"
        )
    }
}

compileOptions {
    sourceCompatibility = JavaVersion.VERSION_17
    targetCompatibility = JavaVersion.VERSION_17
}

// Only needed if using Kotlin
// kotlinOptions {
//     jvmTarget = "17"
// }

}

dependencies { // Add the native library you want to wrap // implementation("com.squareup.okhttp3:okhttp:4.12.0")

// Common dependencies (only add what you need)
// implementation("androidx.core:core-ktx:1.12.0")

}

Checking for Latest Versions

Always use current versions for Gradle plugins and dependencies:

Check latest Android Gradle Plugin version

curl -s "https://maven.google.com/web/index.html" | grep -o 'com.android.tools.build:gradle:[0-9.]*' | head -1

Or check the release notes

open "https://developer.android.com/build/releases/gradle-plugin"

Check latest Kotlin version

curl -s "https://api.github.com/repos/JetBrains/kotlin/releases/latest" | grep '"tag_name"' | cut -d'"' -f4

Step 3: Analyze the Full Dependency Tree

Add your target library to the dependencies and run:

Get full dependency tree

./gradlew app:dependencies --configuration releaseRuntimeClasspath

Get flat list of all dependencies

./gradlew app:dependencies --configuration releaseRuntimeClasspath | grep -E "^[+\\]" | sed 's/.*--- //' | sort -u

For insight into a specific dependency

./gradlew app:dependencyInsight --dependency kotlin-stdlib --configuration releaseRuntimeClasspath

Understanding the output:

  • -> indicates version conflict resolution (e.g., 1.9.10 -> 1.9.21 )

  • (*) indicates the dependency was already shown elsewhere (deduplication)

  • (c) indicates a constraint

Document Your Dependencies

Create a dependency mapping document:

Maven Artifact Version NuGet Package Strategy

androidx.core:core

1.12.0 Xamarin.AndroidX.Core

NuGet

org.jetbrains.kotlin:kotlin-stdlib

1.9.22 Xamarin.Kotlin.StdLib

NuGet

com.example:internal-lib

1.0.0 N/A Bind="false"

org.jetbrains:annotations

23.0.0 N/A Ignore

Step 4: Create the Wrapper Class

Choose Java or Kotlin based on your preference and the native library's language.

Java Wrapper

Create app/src/main/java/com/example/mybinding/DotnetMyBinding.java :

mkdir -p app/src/main/java/com/example/mybinding

cat > app/src/main/java/com/example/mybinding/DotnetMyBinding.java << 'EOF' package com.example.mybinding;

import android.content.Context; import android.util.Log; import android.view.View;

/**

  • Main binding class exposed to .NET

  • Keep method signatures simple for easy marshalling */ public class DotnetMyBinding { private static final String TAG = "DotnetMyBinding"; private static boolean initialized = false;

    // MARK: - Initialization

    /**

    • Initialize the native library

    • Call this from your .NET app's startup */ public static void initialize(Context context, boolean debug) { if (initialized) { Log.w(TAG, "Already initialized"); return; }

      // Initialize your native library here // TheNativeLibrary.configure(context, debug);

      initialized = true; Log.d(TAG, "MyBinding initialized"); }

    /**

    • Check if the library is initialized */ public static boolean isInitialized() { return initialized; }

    // MARK: - Synchronous Methods

    /**

    • Get library version */ public static String getVersion() { return "1.0.0"; }

    /**

    • Process data synchronously

    • @param input The input string to process

    • @return The processed result, or null on failure */ public static String processData(String input) { if (!initialized) { throw new IllegalStateException("Must call initialize() first"); }

      // Process using native library // return TheNativeLibrary.process(input); return "Processed: " + input; }

    // MARK: - Asynchronous Methods (Callback Pattern)

    /**

    • Callback interface for async operations
    • .NET will implement this interface */ public interface DataCallback { void onSuccess(String result); void onError(String error); }

    /**

    • Fetch data asynchronously

    • @param query The query string

    • @param callback Callback for result/error */ public static void fetchDataAsync(String query, final DataCallback callback) { if (!initialized) { callback.onError("Not initialized"); return; }

      // Simulate async operation (replace with real native library call) new Thread(() -> { try { Thread.sleep(100); // Simulate network delay String result = "Result for: " + query; callback.onSuccess(result); } catch (Exception e) { callback.onError(e.getMessage()); } }).start(); }

    /**

    • Callback with multiple result types */ public interface ComplexCallback { void onSuccess(String data, int count, boolean hasMore); void onError(int errorCode, String errorMessage); }

    /**

    • Complex async operation with multiple callback parameters */ public static void performComplexOperation( String input, int options, final ComplexCallback callback) {

      // Perform operation... callback.onSuccess("data", 42, true); }

    // MARK: - View Creation

    /**

    • Create a native view to embed in .NET MAUI

    • @param context Android context

    • @return The native view */ public static View createView(Context context) { // Create and return your native view // return new TheNativeLibrary.CustomView(context);

      View view = new View(context); view.setBackgroundColor(0xFF0066CC); return view; }

    // MARK: - Event Handling

    /**

    • Event listener interface */ public interface EventListener { void onEvent(String eventType, String eventData); }

    private static EventListener eventListener;

    /**

    • Register an event listener */ public static void setEventListener(EventListener listener) { eventListener = listener;

      // Wire up to native library's events // TheNativeLibrary.setEventHandler(event -> { // if (eventListener != null) { // eventListener.onEvent(event.type, event.data); // } // }); }

    /**

    • Remove the event listener */ public static void removeEventListener() { eventListener = null; }

    // MARK: - Configuration

    /**

    • Set configuration options

    • Use simple types that marshal easily */ public static void configure( String apiKey, String apiSecret, int timeout, boolean enableLogging) {

      // Apply configuration to native library Log.d(TAG, "Configured with apiKey: " + apiKey); } } EOF

Kotlin Wrapper (Alternative)

Create app/src/main/kotlin/com/example/mybinding/DotnetMyBinding.kt :

mkdir -p app/src/main/kotlin/com/example/mybinding

cat > app/src/main/kotlin/com/example/mybinding/DotnetMyBinding.kt << 'EOF' package com.example.mybinding

import android.content.Context import android.util.Log import android.view.View

/**

  • Main binding class exposed to .NET

  • Use @JvmStatic for static method access from C# */ object DotnetMyBinding { private const val TAG = "DotnetMyBinding" private var initialized = false

    // MARK: - Initialization

    @JvmStatic fun initialize(context: Context, debug: Boolean) { if (initialized) { Log.w(TAG, "Already initialized") return }

     // Initialize your native library here
 // TheNativeLibrary.configure(context, debug)
 
 initialized = true
 Log.d(TAG, "MyBinding initialized")

    }

    @JvmStatic fun isInitialized(): Boolean = initialized

    // MARK: - Synchronous Methods

    @JvmStatic fun getVersion(): String = "1.0.0"

    @JvmStatic fun processData(input: String): String { check(initialized) { "Must call initialize() first" } return "Processed: $input" }

    // MARK: - Asynchronous Methods

    /**

    • Callback interface - use Java-style interface for better interop */ interface DataCallback { fun onSuccess(result: String) fun onError(error: String) }

    @JvmStatic fun fetchDataAsync(query: String, callback: DataCallback) { if (!initialized) { callback.onError("Not initialized") return }

     Thread {
     try {
         Thread.sleep(100)
         callback.onSuccess("Result for: $query")
     } catch (e: Exception) {
         callback.onError(e.message ?: "Unknown error")
     }
 }.start()

    }

    // MARK: - View Creation

    @JvmStatic fun createView(context: Context): View { return View(context).apply { setBackgroundColor(0xFF0066CC.toInt()) } }

    // MARK: - Event Handling

    interface EventListener { fun onEvent(eventType: String, eventData: String) }

    private var eventListener: EventListener? = null

    @JvmStatic fun setEventListener(listener: EventListener?) { eventListener = listener }

    @JvmStatic fun removeEventListener() { eventListener = null }

    // MARK: - Configuration

    @JvmStatic fun configure( apiKey: String, apiSecret: String, timeout: Int, enableLogging: Boolean ) { Log.d(TAG, "Configured with apiKey: $apiKey") } } EOF

Key Points for Kotlin:

  • Use @JvmStatic on all methods for static access from C#

  • Use object for singleton pattern (maps to static methods)

  • Use Java-style callback interfaces (not Kotlin lambdas)

  • Avoid coroutines in the public API (use callbacks instead)

Add Your Native Library Dependency

Update app/build.gradle.kts dependencies:

dependencies { // The native library you're wrapping implementation("com.thirdparty:awesomelib:1.0.0")

// Common dependencies
implementation("androidx.core:core-ktx:1.12.0")

}

Step 5: Build the AAR

cd android/native

Build release AAR

./gradlew :app:assembleRelease

The AAR will be at:

app/build/outputs/aar/app-release.aar

Verify the AAR contents:

unzip -l app/build/outputs/aar/app-release.aar

Step 6: Create the C# Binding Project

Create the Binding Project with dotnet new

cd ../.. # Back to MyBinding root cd android

Create the binding project using the template

dotnet new androidbinding -n ${BINDING_NAME}.Android.Binding

cd ${BINDING_NAME}.Android.Binding

This creates a properly structured binding project with:

  • Transforms/Metadata.xml

  • For customizing the generated bindings

  • Transforms/EnumFields.xml

  • For enum mappings (if needed)

  • Transforms/EnumMethods.xml

  • For enum method mappings (if needed)

Add the AAR Reference

Edit the generated .csproj to add your AAR and dependencies:

<Project Sdk="Microsoft.NET.Sdk"> <PropertyGroup> <TargetFramework>net9.0-android</TargetFramework> <Nullable>enable</Nullable> <ImplicitUsings>true</ImplicitUsings> <SupportedOSPlatformVersion>21</SupportedOSPlatformVersion> </PropertyGroup>

<!-- Your wrapper AAR --> <ItemGroup> <AndroidLibrary Include="../native/app/build/outputs/aar/app-release.aar" /> </ItemGroup>

<!-- Dependencies: NuGet packages for common libraries --> <ItemGroup> <PackageReference Include="Xamarin.AndroidX.Core" Version="1.12.0.4" /> <PackageReference Include="Xamarin.Kotlin.StdLib" Version="1.9.22.1" />

&#x3C;!-- Add more based on your dependency analysis -->
&#x3C;!-- &#x3C;PackageReference Include="Xamarin.AndroidX.AppCompat" Version="1.6.1.6" /> -->

</ItemGroup>

<!-- Dependencies without NuGet: Include but don't bind --> <ItemGroup> <!-- The library you're wrapping - include but don't generate C# bindings for it --> <!-- <AndroidMavenLibrary Include="com.thirdparty:awesomelib" Version="1.0.0" Bind="false" /> --> </ItemGroup>

<!-- Compile-time only dependencies to ignore --> <ItemGroup> <AndroidIgnoredJavaDependency Include="org.jetbrains:annotations" Version="" /> <!-- <AndroidIgnoredJavaDependency Include="com.google.errorprone:error_prone_annotations" Version="" /> --> </ItemGroup> </Project>

Update Metadata.xml

Edit the generated Transforms/Metadata.xml to customize the bindings:

<metadata> <!-- Rename the Java package to a .NET-friendly namespace --> <attr path="/api/package[@name='com.example.mybinding']" name="managedName">MyBinding</attr>

<!-- Remove obfuscated or internal classes if needed --> <!-- <remove-node path="/api/package[@name='com.example.mybinding']/class[@name='InternalHelper']" /> -->

<!-- Rename parameters from p0, p1 to meaningful names --> <attr path="/api/package[@name='com.example.mybinding']/class[@name='DotnetMyBinding']/method[@name='initialize']/parameter[@name='p0']" name="name">context</attr> <attr path="/api/package[@name='com.example.mybinding']/class[@name='DotnetMyBinding']/method[@name='initialize']/parameter[@name='p1']" name="name">debug</attr>

<attr path="/api/package[@name='com.example.mybinding']/class[@name='DotnetMyBinding']/method[@name='processData']/parameter[@name='p0']" name="name">input</attr>

<attr path="/api/package[@name='com.example.mybinding']/class[@name='DotnetMyBinding']/method[@name='fetchDataAsync']/parameter[@name='p0']" name="name">query</attr> <attr path="/api/package[@name='com.example.mybinding']/class[@name='DotnetMyBinding']/method[@name='fetchDataAsync']/parameter[@name='p1']" name="name">callback</attr> </metadata>

Step 7: Resolve Dependencies

This is the critical step. When you build, .NET 9+ will report dependency errors:

cd android/${BINDING_NAME}.Android.Binding dotnet build

Understanding Dependency Errors

Error Meaning Solution

XA4241 No known NuGet package exists Use AndroidMavenLibrary with Bind="false" or AndroidIgnoredJavaDependency

XA4242 Microsoft maintains a NuGet package Install the suggested NuGet package

Dependency Resolution Decision Tree

For each unsatisfied dependency: │ ├─ Is there a XA4242 suggestion? │ └─ YES → Install the suggested NuGet package │ ├─ Is the dependency needed from C#? │ ├─ YES → Find/create a binding NuGet or create your own binding │ └─ NO → Use AndroidMavenLibrary with Bind="false" │ ├─ Is it a compile-time only dependency (annotations, processors)? │ └─ YES → Use AndroidIgnoredJavaDependency │ └─ Otherwise → Create a separate binding project or include AAR/JAR directly

Finding NuGet Packages for Maven Dependencies

Microsoft-maintained packages include artifact tags. Search by tag:

Search NuGet for a specific Maven artifact

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

Or use curl

curl -s "https://azuresearch-usnc.nuget.org/query?q=tags:artifact=androidx.core:core&#x26;take=10" | jq '.data[] | {id: .id, version: .version}'

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

Practical Example: Complex Library Binding

<Project Sdk="Microsoft.NET.Sdk"> <PropertyGroup> <TargetFramework>net9.0-android</TargetFramework> </PropertyGroup>

<!-- Your wrapper AAR --> <ItemGroup> <AndroidLibrary Include="../native/app/build/outputs/aar/app-release.aar" /> </ItemGroup>

<!-- Dependencies satisfied by NuGet (preferred) --> <ItemGroup> <PackageReference Include="Xamarin.AndroidX.Core" Version="1.12.0.4" /> <PackageReference Include="Xamarin.AndroidX.AppCompat" Version="1.6.1.6" /> <PackageReference Include="Xamarin.Kotlin.StdLib" Version="1.9.22.1" /> <PackageReference Include="Square.OkHttp3" Version="4.12.0.1" /> </ItemGroup>

<!-- The library being wrapped - include but don't bind --> <ItemGroup> <AndroidMavenLibrary Include="com.thirdparty:awesomelib" Version="1.0.0" Bind="false" /> </ItemGroup>

<!-- Dependencies with no NuGet - include but don't bind --> <ItemGroup> <AndroidMavenLibrary Include="com.thirdparty:internal-util" Version="1.0.0" Bind="false" /> </ItemGroup>

<!-- Compile-time only dependencies to ignore --> <ItemGroup> <AndroidIgnoredJavaDependency Include="org.jetbrains:annotations" Version="" /> <AndroidIgnoredJavaDependency Include="com.google.errorprone:error_prone_annotations" Version="" /> <AndroidIgnoredJavaDependency Include="com.google.code.findbugs:jsr305" Version="*" /> </ItemGroup> </Project>

Step 8: Customize Bindings with Metadata

Common Metadata Transforms

Rename Java Packages to .NET Namespaces

<metadata> <attr path="/api/package[@name='com.example.mybinding']" name="managedName">MyBinding</attr> <attr path="/api/package[@name='com.example.mybinding.utils']" name="managedName">MyBinding.Utils</attr> </metadata>

Rename Classes and Methods

<metadata> <!-- Rename a class --> <attr path="/api/package[@name='com.example']/class[@name='Util']" name="managedName">Utilities</attr>

<!-- Rename a method --> <attr path="/api/package[@name='com.example']/class[@name='MyClass']/method[@name='getData']" name="managedName">GetData</attr> </metadata>

Rename Parameters

<metadata> <!-- Rename p0, p1 to meaningful names --> <attr path="/api/package[@name='com.example']/class[@name='MyClass']/method[@name='process']/parameter[@name='p0']" name="name">input</attr> <attr path="/api/package[@name='com.example']/class[@name='MyClass']/method[@name='process']/parameter[@name='p1']" name="name">options</attr> </metadata>

Remove Internal or Obfuscated Types

<metadata> <!-- Remove internal packages --> <remove-node path="/api/package[starts-with(@name, 'com.example.internal')]" />

<!-- Remove obfuscated packages --> <remove-node path="/api/package[@name='a']" /> <remove-node path="/api/package[@name='b']" />

<!-- Remove specific classes --> <remove-node path="/api/package[@name='com.example']/class[@name='InternalHelper']" />

<!-- Remove classes with $ (often internal/anonymous) --> <remove-node path="/api/package/class[contains(@name, '$')]" /> </metadata>

Fix Visibility Issues

<metadata> <attr path="/api/package[@name='com.example']/class[@name='Helper']" name="visibility">public</attr> </metadata>

Examining api.xml for XPath Construction

After building, examine the generated API definition:

cat obj/Debug/net9.0-android/api.xml

This helps construct the correct XPath for your transforms.

Step 9: Build and Verify

dotnet build -c Release

Check for errors and resolve any remaining dependency issues.

Step 10: Use in Your MAUI App

Add Project Reference

In your MAUI app's .csproj :

<ItemGroup Condition="$(TargetFramework.Contains('android'))"> <ProjectReference Include="..\android\MyBinding.Android.Binding\MyBinding.Android.Binding.csproj" /> </ItemGroup>

Initialize in MauiProgram.cs

using Microsoft.Maui.Hosting;

#if ANDROID using MyBinding; #endif

public static class MauiProgram { public static MauiApp CreateMauiApp() { var builder = MauiApp.CreateBuilder(); builder.UseMauiApp<App>();

#if ANDROID // Initialize in a platform-specific lifecycle hook builder.ConfigureLifecycleEvents(lifecycle => { lifecycle.AddAndroid(android => { android.OnCreate((activity, bundle) => { DotnetMyBinding.Initialize(activity, debug: true); }); }); }); #endif

    return builder.Build();
}

}

Implement Callback Interface

#if ANDROID using MyBinding; using Android.Runtime;

// Implement the callback interface public class MyDataCallback : Java.Lang.Object, DotnetMyBinding.IDataCallback { private readonly Action<string> _onSuccess; private readonly Action<string> _onError;

public MyDataCallback(Action&#x3C;string> onSuccess, Action&#x3C;string> onError)
{
    _onSuccess = onSuccess;
    _onError = onError;
}

public void OnSuccess(string result) => _onSuccess(result);
public void OnError(string error) => _onError(error);

} #endif

Use in Your Page

public partial class MainPage : ContentPage { public MainPage() { InitializeComponent(); }

private void OnSyncButtonClicked(object sender, EventArgs e)
{

#if ANDROID // Synchronous method var version = DotnetMyBinding.GetVersion(); var result = DotnetMyBinding.ProcessData("test input"); DisplayAlert("Result", $"Version: {version}\nResult: {result}", "OK"); #endif }

private void OnAsyncButtonClicked(object sender, EventArgs e)
{

#if ANDROID // Async with callback DotnetMyBinding.FetchDataAsync("my query", new MyDataCallback( onSuccess: result => MainThread.BeginInvokeOnMainThread(() => DisplayAlert("Success", result, "OK")), onError: error => MainThread.BeginInvokeOnMainThread(() => DisplayAlert("Error", error, "OK")) )); #endif }

private void OnCreateViewClicked(object sender, EventArgs e)
{

#if ANDROID var context = Platform.CurrentActivity; var nativeView = DotnetMyBinding.CreateView(context);

    // Use with a custom handler or Android.Views.View host

#endif } }

Register Event Listener

#if ANDROID public class MyEventListener : Java.Lang.Object, DotnetMyBinding.IEventListener { private readonly Action<string, string> _onEvent;

public MyEventListener(Action&#x3C;string, string> onEvent)
{
    _onEvent = onEvent;
}

public void OnEvent(string eventType, string eventData)
{
    MainThread.BeginInvokeOnMainThread(() =>
    {
        _onEvent(eventType, eventData);
    });
}

}

// In your page protected override void OnAppearing() { base.OnAppearing();

#if ANDROID DotnetMyBinding.SetEventListener(new MyEventListener((type, data) => { StatusLabel.Text = $"Event: {type} - {data}"; })); #endif }

protected override void OnDisappearing() { base.OnDisappearing();

#if ANDROID DotnetMyBinding.RemoveEventListener(); #endif } #endif

Updating Bindings When Native SDK Changes

Step-by-step Update Process

  1. Update Native Dependency Version

Edit app/build.gradle.kts :

dependencies { implementation("com.thirdparty:awesomelib:2.0.0") // Updated version }

  1. Re-analyze Dependencies

cd android/native ./gradlew app:dependencies --configuration releaseRuntimeClasspath

Compare with your previous dependency mapping and identify:

  • New dependencies added

  • Dependencies with version changes

  • Dependencies removed

  1. Update the Wrapper (If Needed)

Review release notes for the native library and update DotnetMyBinding.java :

  • Add new methods for new APIs

  • Update method signatures for changed APIs

  • Remove deprecated API wrappers

  • Handle any breaking changes

  1. Rebuild the AAR

./gradlew :app:assembleRelease

  1. Update Binding Project Dependencies

Update NuGet packages and Maven dependencies in the binding .csproj based on your new dependency analysis.

  1. Rebuild and Fix Errors

cd ../MyBinding.Android.Binding dotnet build

Address any new XA4241/XA4242 errors.

  1. Update Metadata.xml (If Needed)

If new classes/methods were added to your wrapper, update parameter names and namespace mappings.

  1. Test

Build and run the sample app to verify functionality.

Handling Native Libraries (.so files)

Some Android libraries include native code (.so files). If you get java.lang.UnsatisfiedLinkError , ensure native libraries are properly included.

Including .so Files

For AAR files, .so files are usually included automatically. For JAR files, add them manually:

<ItemGroup> <AndroidNativeLibrary Include="libs\arm64-v8a\libmynative.so"> <Abi>arm64-v8a</Abi> </AndroidNativeLibrary> <AndroidNativeLibrary Include="libs\armeabi-v7a\libmynative.so"> <Abi>armeabi-v7a</Abi> </AndroidNativeLibrary> <AndroidNativeLibrary Include="libs\x86_64\libmynative.so"> <Abi>x86_64</Abi> </AndroidNativeLibrary> </ItemGroup>

Manually Loading Native Libraries

Sometimes you need to explicitly load the native library:

// In your Android initialization code Java.Lang.JavaSystem.LoadLibrary("mynative");

Troubleshooting

Build Errors

"Java dependency 'X' is not satisfied" (XA4241/XA4242)

Missing transitive dependency.

Solution: Follow the dependency resolution decision tree:

  • If XA4242 suggests a NuGet package → Install it

  • If you don't need the API in C# → Use AndroidMavenLibrary with Bind="false"

  • If it's compile-time only → Use AndroidIgnoredJavaDependency

"Type is defined multiple times"

Duplicate classes from including the same dependency twice.

Solution: Check for overlap between NuGet packages and AAR contents. Use Bind="false" or remove redundant references.

"Class 'X' does not implement interface member 'Y'"

Covariant return types or interface implementation issues.

Solution: Add custom implementation in Additions/ folder:

// Additions/MyClassAdditions.cs namespace Com.Example { public partial class MyClass { Java.Lang.Object SomeInterface.SomeMethod() { return RawSomeMethod(); } } }

"Cannot find symbol" / "Package does not exist"

Wrapper code references classes not available.

Solution: Verify all dependencies are declared in build.gradle.kts and rebuild the AAR.

Gradle Build Failures

Clean and rebuild

./gradlew clean ./gradlew :app:assembleRelease --stacktrace

Runtime Errors

Java.Lang.NoClassDefFoundError

Missing dependency at runtime.

Solution: A required dependency was ignored. Remove it from AndroidIgnoredJavaDependency and properly satisfy it.

Java.Lang.UnsatisfiedLinkError

Native library (.so) not loaded.

Solution:

  • Ensure .so files are included in the AAR

  • Call JavaSystem.LoadLibrary() if needed

  • Check ABI compatibility (arm64-v8a, armeabi-v7a, x86_64)

Java.Lang.IllegalStateException: Must call initialize() first

Library not initialized.

Solution: Ensure you call DotnetMyBinding.Initialize() before using other methods.

Callback Not Invoked

Causes & Solutions:

  • Callback on wrong thread - Use MainThread.BeginInvokeOnMainThread() for UI updates

  • Callback garbage collected - Store a strong reference to the callback object

  • Exception in native code - Add try/catch in wrapper and check logcat

IntelliSense Issues

IntelliSense shows errors but project compiles: This is normal for binding projects. The binding generator doesn't use source generators. Build the project first and IntelliSense will catch up (though some errors may persist).

Quick Reference

Wrapper Design Guidelines

Type Mapping

Keep method signatures simple with types that marshal easily:

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

Callback Interface Pattern

// Java - define a simple callback interface public interface DataCallback { void onSuccess(String result); void onError(String error); }

// C# - implement the generated interface public class MyCallback : Java.Lang.Object, DotnetMyBinding.IDataCallback { private readonly Action<string> _onSuccess; private readonly Action<string> _onError;

public MyCallback(Action&#x3C;string> onSuccess, Action&#x3C;string> onError)
{
    _onSuccess = onSuccess;
    _onError = onError;
}

public void OnSuccess(string result) => _onSuccess(result);
public void OnError(string error) => _onError(error);

}

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

Complete Script: Create Android Binding Project

#!/bin/bash set -e

Configuration

BINDING_NAME="${1:-MyBinding}" PACKAGE_NAME="${2:-com.example.mybinding}" MIN_SDK="${3:-21}" GRADLE_VERSION="${4:-9.2.1}" AGP_VERSION="${5:-8.2.2}"

echo "Creating Android binding project: ${BINDING_NAME}" echo " Package: ${PACKAGE_NAME}" echo " Min SDK: ${MIN_SDK}" echo " Gradle: ${GRADLE_VERSION}"

Create directory structure

mkdir -p ${BINDING_NAME}/android/native/app/src/main/java/${PACKAGE_NAME//.//} mkdir -p ${BINDING_NAME}/android/${BINDING_NAME}.Android.Binding/Transforms mkdir -p ${BINDING_NAME}/sample/MauiSample

cd ${BINDING_NAME}/android/native

Download and setup Gradle Wrapper (no global Gradle install needed)

echo "Setting up Gradle Wrapper..." GRADLE_DIST_URL="https://services.gradle.org/distributions/gradle-${GRADLE_VERSION}-bin.zip" WRAPPER_JAR_URL="https://raw.githubusercontent.com/gradle/gradle/v${GRADLE_VERSION}/gradle/wrapper/gradle-wrapper.jar"

mkdir -p gradle/wrapper cat > gradle/wrapper/gradle-wrapper.properties << EOF distributionBase=GRADLE_USER_HOME distributionPath=wrapper/dists distributionUrl=https://services.gradle.org/distributions/gradle-${GRADLE_VERSION}-bin.zip networkTimeout=10000 validateDistributionUrl=true zipStoreBase=GRADLE_USER_HOME zipStorePath=wrapper/dists EOF

Create gradlew script

cat > gradlew << 'GRADLEW_EOF' #!/bin/sh

Gradle Wrapper script - downloads Gradle if needed

exec java -jar "$(dirname "$0")/gradle/wrapper/gradle-wrapper.jar" "$@" GRADLEW_EOF chmod +x gradlew

Download gradle-wrapper.jar

curl -sL -o gradle/wrapper/gradle-wrapper.jar
"https://raw.githubusercontent.com/gradle/gradle/v${GRADLE_VERSION}/gradle/wrapper/gradle-wrapper.jar" 2>/dev/null ||
echo "Note: Could not download gradle-wrapper.jar. Run './gradlew' to auto-download."

Create settings.gradle.kts

cat > settings.gradle.kts << EOF pluginManagement { repositories { google() mavenCentral() gradlePluginPortal() } }

dependencyResolutionManagement { repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS) repositories { google() mavenCentral() } }

rootProject.name = "${BINDING_NAME}Native" include(":app") EOF

Create root build.gradle.kts

cat > build.gradle.kts << EOF plugins { id("com.android.library") version "${AGP_VERSION}" apply false } EOF

Create app build.gradle.kts

cat > app/build.gradle.kts << EOF plugins { id("com.android.library") }

android { namespace = "${PACKAGE_NAME}" compileSdk = 34

defaultConfig {
    minSdk = ${MIN_SDK}
}

buildTypes {
    release {
        isMinifyEnabled = false
    }
}

compileOptions {
    sourceCompatibility = JavaVersion.VERSION_17
    targetCompatibility = JavaVersion.VERSION_17
}

}

dependencies { // Add your native library dependency here // implementation("com.thirdparty:awesomelib:1.0.0")

implementation("androidx.core:core:1.12.0")

} EOF

Create Java wrapper

cat > app/src/main/java/${PACKAGE_NAME//.//}/Dotnet${BINDING_NAME}.java << EOF package ${PACKAGE_NAME};

import android.content.Context; import android.util.Log;

public class Dotnet${BINDING_NAME} { private static final String TAG = "Dotnet${BINDING_NAME}"; private static boolean initialized = false;

public static void initialize(Context context, boolean debug) {
    if (initialized) return;
    initialized = true;
    Log.d(TAG, "${BINDING_NAME} initialized");
}

public static boolean isInitialized() {
    return initialized;
}

public static String getVersion() {
    return "1.0.0";
}

public interface DataCallback {
    void onSuccess(String result);
    void onError(String error);
}

public static void fetchDataAsync(String query, DataCallback callback) {
    if (!initialized) {
        callback.onError("Not initialized");
        return;
    }
    callback.onSuccess("Result for: " + query);
}

} EOF

cd ../..

Create binding project using dotnet new template

dotnet new androidbinding -n ${BINDING_NAME}.Android.Binding -o ${BINDING_NAME}.Android.Binding

Add the AAR reference to the generated csproj

The template creates a basic project, we need to add our specific items

cat >> ${BINDING_NAME}.Android.Binding/${BINDING_NAME}.Android.Binding.csproj << EOF

<!-- Your wrapper AAR --> <ItemGroup> <AndroidLibrary Include="../native/app/build/outputs/aar/app-release.aar" /> </ItemGroup>

<!-- Dependencies --> <ItemGroup> <PackageReference Include="Xamarin.AndroidX.Core" Version="1.12.0.4" /> </ItemGroup>

<ItemGroup> <AndroidIgnoredJavaDependency Include="org.jetbrains:annotations" Version="*" /> </ItemGroup> EOF

Fix the csproj by moving the added items inside the Project element

(The template ends with </Project>, so we need to insert before it)

sed -i '' 's/<\/Project>//' ${BINDING_NAME}.Android.Binding/${BINDING_NAME}.Android.Binding.csproj echo "</Project>" >> ${BINDING_NAME}.Android.Binding/${BINDING_NAME}.Android.Binding.csproj

Update Metadata.xml with our namespace mapping

cat > ${BINDING_NAME}.Android.Binding/Transforms/Metadata.xml << EOF <metadata> <attr path="/api/package[@name='${PACKAGE_NAME}']" name="managedName">${BINDING_NAME}</attr> </metadata> EOF

cd ..

echo "" echo "✅ Created ${BINDING_NAME} Android binding project!" echo "" echo "Next steps:" echo " 1. cd ${BINDING_NAME}/android/native" echo " 2. Add your native library to app/build.gradle.kts" echo " 3. ./gradlew :app:assembleRelease" echo " 4. cd ../${BINDING_NAME}.Android.Binding" echo " 5. dotnet build" echo " 6. Resolve any dependency errors"

Save as create-android-binding.sh and run:

chmod +x create-android-binding.sh

Basic usage (uses defaults)

./create-android-binding.sh MyAwesomeBinding

Full customization

./create-android-binding.sh MyAwesomeBinding com.mycompany.mybinding 21 9.2.1 8.2.2

^name ^package ^sdk ^gradle ^agp

Tip: The script sets up Gradle Wrapper, so you don't need Gradle installed globally. The first ./gradlew run will download the correct Gradle version automatically.

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 - Check for latest AGP version

  • Gradle Releases - Check for latest Gradle version

  • Kotlin Releases - Check for latest Kotlin version

Templates and Examples

  • Maui.NativeLibraryInterop Repository

  • Xamarin.AndroidX Repository

Related Skills

  • See docs/android-bindings-guide.md for comprehensive reference

  • See ios-slim-bindings for iOS bindings

Output Format

When assisting with Android slim bindings, provide:

  • Dependency analysis - Full dependency tree and NuGet mapping strategy

  • Project structure - File/folder layout for the binding

  • Wrapper code - Complete Java or Kotlin implementation

  • Gradle configuration - build.gradle.kts with dependencies

  • C# binding project - .csproj with proper dependency references

  • Metadata transforms - Metadata.xml for namespace and parameter renaming

  • Usage examples - How to call the binding from MAUI/C#

  • 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

Source Transparency

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