How to call C++ library code from Xamarin Forms project - DllNotFoundException

Question

I have Xamarin forms cross platform project - Android, iOS and UWP. I have a C++ DLL project from which I have created a DLL. My intention is to integrate/call these C++ code within Xamarin forms cross. I know how to call C++ from C# Console application with DLLImport assembly directive, but on Xamarin I get DllNotFoundException.

So where the DLL file has to be copied or referenced for this to work?

• I have copied .dll and other files in Debug folder(where the current project is trying to reference DLL. (I referred this video and it works for C# console )
• Even tried Add Reference to DLL file - gives error cannot add file
• Adding C++ DLL project to same solution and referencing it is giving System.DllNotFoundException.
• Created cross platform shared C++ (I think this is comes in NDK)library project but still get the same issue - DllNotFoundException.

Code :

1. C++

   #include "stdafx.h"
   
   extern "C"{
   
       __declspec(dllexport) int add(int a, int b)
       {
           return a + b;
       }
       __declspec(dllexport) int subtract(int a, int b)
       {
           return a - b;
       }
   }
   

2. App.xaml.cs or any other .Net standard shared project

   public partial class App : Application
   {
   [DllImport("Dll1.dll", CallingConvention = CallingConvention.Cdecl)]
   public static extern int subtract(int a, int b);
   
   
   [DllImport("Dll1.dll", CallingConvention = CallingConvention.Cdecl)]
   public static extern int add(int a, int b);
   
   [DllImport("XamCppDll.Shared.so")]
   public static extern int cpp_GetValue(int a, int b);
   
   public App()
   {
       InitializeComponent();
   
       int x = 100;
       int y = 15;
       int z = 0;
       int z1 = 0;
       try
       {
           cpp_GetValue(5, 6);//XamCppDll.Shared.so
           z = subtract(x, y);//Dll1.dll
           z1 = add(x, y);//Dll1.dll
       }
       catch (System.Exception ex1)
       {
           Debug.WriteLine(ex1.Message);
       }
   
       Debug.WriteLine(" {0} - {1} = {2}", x, y, z);
       Debug.WriteLine(" {0} + {1} = {2}", x, y, z1);
   
       MainPage = new MainPage();
   
   }
   

   }

Project structure

Tutorials/codes referred

https://blog.xamarin.com/build-and-debug-c-libraries-in-xamarin-android-apps-with-visual-studio-2015/

https://blogs.msdn.microsoft.com/vcblog/2015/02/23/developing-xamarin-android-native-applications/

Answer 1

I figured this issue after spending so much time and testing.

In order load library the C++ should be packed as .so(Shared Object) extension within Android.

This file has to be packed and placed within .apk of the android.

• Create and compile Xamarin C++ cross platform shared library project.
• Compile Android specific linked project to get different architecture .so : x86 , x64 , ARM, ARM64 - these correspond to different ABIs too.

• Link .so with android ABI

  1. Option 1 : create lib/abi_name folder and set .so file as AnroidNativeLibrary where abi_name can be

     • armeabi
     • armeabi-v7a
     • arm64-v8a
     • x86
     • x86_64
       Important note : In order to .so file to being copied to APK folder structure should be exactly as above. If device supported architecture is arm64-v8a but only x86 folder is created then .so file wont be available within APK and we still get DllNotFoundException.

  2. Option 2: Add .so file into android project with Build action AndroidNativeLibrary with <ABI> tag specifier. In Android .csproj file add as below.

     <ItemGroup>
     <AndroidNativeLibrary Include="path/to/libFile.so">
         <Abi>armeabi</Abi>
     </AndroidNativeLibrary>
     

I have added Github repository XamarinNDKSample for this if anyone wants working code.