DllImport not working on Docker - DllNotFoundException

Question

I have a project developed with .NET Core and C#, running on Docker, that has to call a few functions on a DLL developed with C++. The problem is: when I run my project without Docker, on Windows using Visual Code, the code runs smoothly, but when I run on Docker, on a Linux container, the code throws an error when trying to execute the DLL function.

I already tried copying the .dll file to the /lib folder, changing it to the parent folder of the project and none of that worked. I started to doubt that the problem is that the file is not found and, by doing some research, I saw that it could be related to the file permissions, so I ran chmod a+wrx on the .dll file, also no success.

This is my Dockerfile configuration:

FROM mcr.microsoft.com/dotnet/core/aspnet:2.2 AS base
WORKDIR /app
EXPOSE 80
RUN apt-get update \
    && apt-get install -y --allow-unauthenticated \
        libc6-dev \
        libgdiplus \
        libx11-dev \
     && rm -rf /var/lib/apt/lists/*
RUN apt-get update \
    && apt-get install -y poppler-utils

FROM mcr.microsoft.com/dotnet/core/sdk:2.2 AS build-env
WORKDIR /app
COPY . .
RUN dotnet restore --configfile Nuget.config -nowarn:msb3202,nu1503
RUN dotnet publish -c Release -o ./out 
    
FROM base AS final
WORKDIR /app
COPY --from=build-env /app/out .
ENTRYPOINT ["dotnet", "MdeGateway.dll"]

This is the code that tries to access the DLL function:

[DllImport("MyDll.dll")]
private static extern int dllfunction(Int32 argc, IntPtr[] argv);

public static void CallDll(string[] args)
{
    IntPtr[] argv = ArrayToArgs(args);
    dllfunction(args.Length, argv);

    FreeMemory(args, argv);
}

The error occurs when the line 'dllfunction(args.Length, argv);' is executed.

The exact message is:

"Unable to load shared library 'MyDll.dll' or one of its dependencies. In order to help diagnose loading problems, consider setting the LD_DEBUG environment variable: libMyDll.dll: cannot open shared object file: No such file or directory"

Also, if someone can teach me how to set the LD_DEBUG environment variable I would appreciate it.

Answer 1

I have a project developed with .NET Core and C#, running on Docker, that has to call a few functions on a DLL developed with C++. The problem is: when I run my project without Docker, on Windows using Visual Code, the code runs smoothly, but when I run on Docker, on a Linux container, the code throws an error when trying to execute the DLL function.

If I am reading this right, you have a C++ application that you compiled to a .dll (on Windows). You can DllImport this .dll on Windows, but not on Linux (container). Is that right?

Are you aware that C++ code compiled into a .dll (shared library) is a Windows-specific thing? Unmanaged code is architecture and platform specific. An unmanaged .dll compiled on x64 won't run on arm64. A unmanaged .dll compiled on Windows wont run on Linux.

Linux (and Linux containers, such as in docker) can't use a .dll built from unmanaged code on Windows. Linux needs the unmanaged (C++) code to be compiled into a shared library (.so file) for DllImport (and the underlying dlopen calls) to work on Linux. Ideally on the same platform as the container it will be running in.

The mono docs cover an (one particular) implementation of DllImport and give more background on how this works on Linux:

https://www.mono-project.com/docs/advanced/pinvoke/

(But keep in mind that Mono != .NET Core. It should still give you some more background information.)