Develop Reference

How to link a .so file to a managed DLL with mono

I'm writing a program for embedded linux on an ARM processor in .net that uses mono to execute. I have found that I can simply compile to either x86 or x64 architecture on my PC, copy the Debug directory over to linux, and run the program using mono myProgram.exe. The program is working perfectly like this and mono seems to take care of the architecture mismatch. This was true until today when I tried to incorporate an external native DLL.
I have both an x86 and x64 bit architecture of the external DLL and both work on the desktop environment fine. I have a shim class that uses [DllImport] to load the extern functions. However, when I try this on embedded linux with Mono I am getting a BadImageFormat exception. My guess is that Mono somehow transitions the compiled DLLs in the exe at start up but doesn't do the same for the external.
Some other info:
1. I don't have source for the native external library. I have x86, x64, and a .so library.
2. I have tried making a manage C++ shim that accesses the .so but can't get it to compile on the desktop since windows doesn't recognize the .so file.
Some thoughts:
1. Is there a way to embed the native DLL into a managed so that Mono will transition the native DLL too?
2. Can I link the .so file to a managed C++ project?
3. Is there a way to tell Mono to incorporate that Dll during execution?
Ultimately I'm looking for a solution that is wrapped in the exe that allows me to simply run it like I did before and control the native system.

If you have all the .dll and .so files for Windows and Linux, you can probably use dll maps feature of Mono. (https://www.mono-project.com/docs/advanced/pinvoke/dllmap/) which are used to map Windows dll names to Linux so names.
.Net does not recognize this, but Mono does.

How to access native APIs from Mono (command-line) app?

Is it possible (and if so, how) to call an external API from an ordinary command-line Mono app?
For example: I've installed Mono on my Mac via this page and have been happily writing command-line apps, but now want to access some simple CoreFoundation API. I really don't want to switch to Xamarin.Mac just for that. Is this possible?
As a closely related question, can I create my own shared library exposing a C API, and somehow invoke this from my Mono app?

The Mono Docs cover P/Invoke here : http://www.mono-project.com/docs/advanced/pinvoke/
Citing:
The Common Language Infrastructure (CLI) is designed to make it “easy” to interoperate with existing code. In principle, all you need to do is create a DllImport function declaration for the existing code to invoke, and the runtime will handle the rest. For example:
[DllImport ("libc.so")]
private static extern int getpid ();
Because you specifically asked about Mac:
Note: Mono uses GLib to load libraries, and GLib has a bug on Mac OS X where it doesn’t use a .dylib extension, but instead uses the Unix .so extension. While this should eventually be fixed, the current workaround is to write a .config file which maps to the .dylib file, e.g. [and so on ...]

Does Xamarin C# apps work without requiring Mono compiler?

I mean, if I send an apk developed in Xamarin to my friend to install, does he needs Mono framework on his phone or it will install and run without any 3rd party application?

On Android, Xamarin ships a fully functional implementation of the .NET runtime, called Mono, bundled with your app so that your code executes with all of the power of C# and .NET, including JIT-compilation, memory management, reflection, and the .NET base class libraries.
Source
Yes, that should work fine without requiring Mono to be installed on your friend's device.

Can a C# program be cross-platform?

I'm a newbie to programming, and I'm considering using C# to write a VERY simple program that simply edits a text file.
Will it work on Macs and Linux? What will work?
I want to know what makes a program cross-platform?

C# can be compiled for and ran on multiple platforms (Windows, Linux, macOS) by using .NET Core.
Previously, cross-platform development using C# could only be done by using Mono which is a third-party implementation however it is now recommended to use .NET Core as it is developed by Microsoft.

Now C# is fully cross platform and has variety of frameworks and run times.
Java is a language + framework. But C# is just a modern language. C# language features are very powerful, easy to learn. No confusion between framework and language.
C# can be used in following frameworks
.Net core (Multi platform supports Linux, Windows and Mac), mostly used to develop restful services, web applications and enterprise systems
.Net Framework (Windows only), Same as .net core, but has more windows only features
Unity - One of most used game engine to develop games for PC, Sony, Xbox devices etc..
Xamarin - To develop native Mobile apps (Android, IOS, Mac)
.Net standard - Create a library in .net standard use in all above applications
Java - Code once, Ship same file to all platforms
C++, C, Rust - Code once, Compile for specific platform
C# - It is just a language, pick your framework according to requirement.
Most popular applications like Chrome, Firefox uses C++, Rust and they are compiled for each platform. But still they share the majority of code between these platforms. Some games use C++ to share the code between IOS and Android.
For new developers its good to start with NodeJS and C#. Both are easy and can do any type of applications.

It's hard to answer something like that to somebody, which states that he just starts programming, but I'll try to give you an (somewhat simplified) overview: C# is a language, which is compiled into an executable program. The language itself is portable between plattforms, as long as you have a compiler on each target plattform. On Windows you have the "default" .Net implementation and on Linux/Mac you have Mono. To do something useful - like editing files - you have to use some libraries. There is a standard library which is available on each plattform. As long as you use only that functionality, your program will be cross plattform. Obviously is will not be cross plattform, as soon as you use some library specific to a certain plattform. Editing files is part of the standard library, so you can implement such simple tools in a cross plattform way. When choosing a book, you should look for books which focus on the "raw" language and not on Visual Studio or other specific tools.

A new open source release from Microsoft called CoreCLR is cross-platform and can now be used for this
The CoreCLR repo contains the complete runtime implementation for .NET Core. It includes RyuJIT, the .NET GC, native interop and many other components. It is cross-platform, with multiple OS and CPU ports in progress.
-from the CoreCLR repo

I would not recommend c# as the top choice for multiplatform coding. I would recommend python instead it is very easy to run even as a script and works perfectly on all platforms. If you insist on c# be prepared to deal with the same problem explained by Justin Ethier. Python on the other hand, is the same for all platforms, it is super powerful fast and even has wrappers ready to connect to other languages.

C# is cross-platform thanks to .NET Core, but without a cross-platform User Interface library you can only make very basic console programs with it. In order to make cross-platform apps for Windows, Mac and Linux you need a cross-platform User Interface library like Nevron Open Vision

Yes, C# is fully cross-platform thanks to .NET Core.
.NET Core is a free and open-source, managed computer software framework for Windows, Linux, and macOS operating systems. It is the cross-platform successor to the .NET Framework which only run on Windows. The project is primarily developed and supported by Microsoft (through the .NET Foundation).

C# is cross platform and you can run your program in Windows/Linux and Mac. For Linux, though, you need to run it using mono

How does Mono work

I have used C# in Visual Studio with .NET, and I have played around a little with Mono on openSUSE Linux, but I don't really understand how it works.
If I write an app in Windows on .NET, how does this relate to Mono? I can't just execute an a Windows .exe file on Linux without Wine, so it doesn't help me execute apps developed in Windows.
Is the purpose purely to have a .NET library equivalent on Linux (and others) to make cross platform development easier? For example, if I was a business and wanted to reach Linux customers, but really wanted to use .NET, then Mono should be my choice? Or is there something more that I'm missing?

This is an old question (with an already selected answer) but I do not believe the question has really been answered well.
First, a little background...
How does .NET work?
A traditional Windows .EXE file is a binary file that represents a series of machine language instructions that your computer understands and that makes calls into the Win32 API which are parts of Windows that deliver services that applications can take advantage of. The machine language used is very specific to your kind of computer and the Win32 calls make the executable very dependent on Windows. A .NET executable is not like that.
It is important to realize that a .NET executable (.EXE file) is not actually a native Windows application. Windows itself does not understand how to run the code in a .NET executable. Your computer does not understand it either.
Much like Java, a .NET application is made up of instructions in a language called CIL (Common Intermediate Language) that you can think of as the machine language for an idealized computer that does not really exist. In .NET, the software implementation of this idealized machine is called the Common Language Runtime (CLR). The equivalent in the Java world is called the Java Virtual Machine (JVM). In Java, the equivalent to CIL is called Java bytecode. CIL is sometimes called MSIL (Microsoft Intermediate Language).
CIL is designed to run on the CLR (an idealized machine) but is otherwise platform independent, which means that the CIL does not care what kind of computer you have or what operating system you are running.
Just as you need a native version of the Java JVM on each platform on which you want to run Java, you need a native version of the CLR to run .NET CIL executables. The CLR is a native Windows application just like the traditional Win32 EXE files described above. The CLR itself is specific to the Windows implementation and computer architecture on which it was designed to run.
It does not matter what .NET language you start with (C#, VisualBasic, F#, IronPython, IronRuby, Boo, etc.), they all get compiled down to CIL bytecode. You can easily "disassemble" a CIL program into a form of object-oriented assembly language that is easily readable by humans. You can write a program in CIL directly yourself but few people do.
On Windows, the CLR compiles this CIL code Just-in-Time (JIT) right when you run the executable--just before the code is actually run. This means that the CIL bytecode is converted (compiled) to actual machine code that runs natively on your computer. This part of the CLR is called the JIT compiler or often just the JIT.
To date, Microsoft has released four versions of the CLR: 1.0, 1.1, 2.0, and 4.0. You need to have the right version of the CLR installed on your machine if you want to run .NET executables targeting that runtime. The CLR 2.0 supports .NET 2.0, 3.0, and 3.5 applications. For other versions of .NET, the .NET version maps cleanly to the CLR version.
In addition to the JIT/CLR, .NET provides a host of libraries (assemblies) that make up the rest of the .NET framework and that provide a host of capabilities and services that .NET applications can call upon. The great majority of these assemblies are pure CIL code which runs on the CLR. On Windows, a some make calls into the Win32 API as well. When you install .NET, you are installing the CLR, the class libraries (framework), and a bunch of development tools. Each version of the CLR generally requires a complete set of these "framework" assemblies. Some versions of .NET (eg. 3.0 and 3.5) added additional framework assemblies without updating the CLR or the existing assemblies associated with that CLR.
The Portable Executable (PE) file format that a Windows .EXE file is delivered in contains a header that describes the executable and identifies the file as a .NET file or a native Win32 file. When Windows tries to run a .NET file, it sees this header and automatically invokes the CLR on your behalf. This is why .NET EXE files appear to run natively on Windows.
Ok, so how does Mono work?
Mono implements the CLR on Linux, Mac, and other platforms. The Mono runtime (the CLR) is a native application written mostly in the C language and compiled down to machine language code for the computer system on which is designed to run. Like on Windows, the Mono runtime is specific to the Operating System and kind of machine you are using.
Just like on Windows, the Mono runtime (the CLR) compiles the CIL bytecode in your .NET executable Just-in-time to native code that your computer can understand and execute. In this way, a .NET file is just as "native" to Linux as it is to Windows.
To port Mono to a new architecture you need to port the JIT/CLR. This is just like porting any native application to a new platform.
How well .NET code runs on Linux or Mac is really just a question of how well the CLR is implemented on these systems. In theory, the Mono CLR could execute .NET code on these systems much better than the MS version of .NET does on Windows. In practice, the MS implementation is generally superior (though not in all cases).
In addition to the CLR, Mono provides most of the rest of the libraries (assemblies) that make up the .NET framework. Just as with the Microsoft version of .NET (in fact more so) the Mono assemblies are provided as CIL bytecode. This makes it possible to take a *.dll or *.exe file from Mono and run it unmodified on Windows, Mac, or Linux as CIL is the "native" language of the CLR implementations on these systems.
Just like on Windows, Mono supports multiple versions of the CLR and the associated assemblies:
Very early versions of Mono (before 1.2?) only supported CLR 1.0 or 1.1.
Mono did not support big chunks of the 2.0 framework until it's own 2.0 version.
Mono versions up to version 2.4 supported both CLR 1.1 and CLR 2.0 applications.
Starting with Mono 2.6, CLR 4.0 was added but CLR 2.0 was still the default.
Starting with Mono 2.8 the CLR 4.0 became the default and the CLR 1.1 is no longer supported.
Mono 2.10 continues to use the CLR 4.0 as default and also to support the CLR 2.0.
Just like the real .NET (but in far fewer cases) there are some Mono assemblies that call into native libraries. In order to make the System.Drawing assembly work on Mono, the Mono team wrote a Linux program to simulate the GDI+ portion of the Win32 API on Linux. This library is called 'libgdiplus'. If you compile Mono from source, you will notice that you need to build this 'libgdiplus' file before you can build 'mono'. You do not need 'libgdiplus' on Windows because the GDI+ portion of the Win32 API is already part of Windows. A full port of Mono to new platforms requires this 'libgdiplus' library to be ported as well.
In areas where the design of the .NET library is overly influenced by the design of Windows, and a poor fit for systems like Mac or Linux, the Mono team has written extensions to the .NET framework. The Mono extensions are also just CIL bytecode and generally work just fine on .NET.
Unlike on Windows, Linux generally does not detect .NET executables and launch the CLR by default. The user must usually run the CLR directly by typing 'mono appname.exe' or something similar. Here 'mono' is the application that implements the CLR and 'appname.exe' is the EXE file that contains the .NET code to be executed.
To make things easier for users, Mono applications are often wrapped in a shell script that launches the CLR. This hides the fact that the CLR is being used just as in Windows. It is also possible to tell Linux to launch the CLR when a file using the PE file format is encountered. This is usually not done as the PE file format is also used for native Win32 Windows executables which of course the CLR (Mono) does not support.
There is no technical reason why a PE launcher could not be used by Linux which then launches either a system that understands native Windows code (like Wine) or the CLR (Mono) as appropriate. This has simply not been done to my knowledge.
Back and forth
Any .NET code that sticks to "fully managed" code, which means it does not call into non-.NET code, should work fine on Mono on all platforms. I routinely use compiled .NET assemblies from Windows (for which I do not have the code) on Linux and Mac.
I can also take any code that I compile on Mono and run that on .NET on Windows. I can provide a client some code I compiled with Mono and not worry if he is on 32-bit or 64-bit Windows for example. The client does need to have the right version of .NET (the right CLR) installed fo course. CLR 2.0 has been around for a very long time and you can bet almost all Windows users have it installed. The Mono compilers and other code are also just CIL executables and so they run fine on Windows if you like.
Mono compatibility is good enough that large chunks of actual Microsoft code, like ASP.NET MVC, can be taken (where legal to do so) from the actual MS version of .NET and run on Mac or Linux. In general, the Mono team has done a great job of implementing both the CLR and the rest of the framework (class libraries/assemblies).
ASP.NET
On Windows, the Internet Information Server (IIS) knows how to call into the CLR to execute .NET as part of a web application. On Linux/Mac there is an Apache module (mod_mono) that provides similar capabilities to the Apache webserver. This application is written in C and must also be ported to new architectures.
Porting Mono
This discussion has identified parts of Mono that are built as "native" executables and must exist on a system on which you want to run .NET applications.
The CLR (including JIT compiler) - generally known as Mono
libgdiplus (for systems which do not natively support the GDI+ API [only Windows does])
mod_mono (to allow Apache to invoke the CLR for .NET web applications)
These three components, with the addition of the class libraries, provide a .NET environment that looks "native" to the .NET executable files you need to run.
That is how Mono works.

A Windows EXE contains multiple "parts". Simplified, the .net Code (=MSIL) is only a Part of the EXE, and there is also a "real" native Windows Part inside the EXE that serves as some sort of launcher for the .net Framework which then executes the MSIL.
Mono will just take the MSIL and execute it, ignoring the native Windows Launcher stuff.
Again, this is a simplified overview.
Edit: I fear my understanding of the deep deep details is not good enough for really much detail (I know roughly what a PE Header is, but not really the details), but i found these links helpful:
NET Assembly Structure – Part II
.NET Foundations - .NET assembly structure

You can in fact run a .NET .exe file with Mono on Linux. This does not require Wine. In fact, Mono compiles programs to .exe files, which can run either with Mono on Linux or as an executable on Windows.

Mono is an open-source implementation of Microsofts .NET CLR (Common Language Runtime). This is what runs part of .NET programs which are not in native code but in CIL (Common Intermediate Language), a language and machine-neutral intermediate language. The Runtime takes that intermediate code and translates it into machine code.
At the current state of Mono, you can take .NET programs that use the main parts of .NET (mscorlib.dll) and run them everywhere Mono runs, not just Windows.

But as it is mentioned that Mono is open source and you can't just rely that it will be the full .NET implementation, it has some controls that are not working, you must be also careful with P/Invokes that your application will use, for e.g your application will communicate with MCI (Multimedia Controller Interface) under win32. But I was using mono writing GTK# Applications also, but I've also used my Windows applications that worked without any recompilation as mentioned our fellow programmers above, that is, mono is an open source alternative of Microsoft's .NET, and by default if you are building either WinForms or Gtk# applications mono will compile and will create an .exe assembly for each file, and of course if you want it will create an Dynamic Link Library (DLL), almost as it is done in .NET. Just for suggestion try writing some Gtk# (with MonoDevelop IDE which has its built-in gui designer called stetic). And of course mono can be a great replacement for Web Services that you can create them on .NET and you can host them on Apache (because Linux hosting nowadays are more cheap than Windows ones) web services and other asp.net apps will work under apache with a mod_mono module that must be included in apache.
A little bit out of topic but I just wanted to tell you a sneak-peek from my experience.

Also you can take a look to MoMA (if your goal is to port applications from Win to Lin).
The Mono Migration Analyzer (MoMA)
tool helps you identify issues you may
have when porting your .Net
application to Mono. It helps pinpoint
platform specific calls (P/Invoke) and
areas that are not yet supported by
the Mono project.
MoMA
Here is a webapp that compares the types of the BCL already implemented by Mono and the .NET Framework 3.5
http://mono.ximian.com/class-status/2.0-vs-3.5/index.html

To further Michael's response, I believe you will have to recompile on Mono for the app to run on the Mono runtime. Exceptions may exist. I've only played around with Mono just a bit, and I've always re-compiled the source. I've never tried to run a .NET app directly on Mono.
Also, Mono 1.9 is supposed to be fully .NET 2.0 compliant. Mono Olive and Moonlight are supposed to add .NET 3.0 (less WPF) and Silverlight functionality.

It might help you, How does Mono's C# compiler work? and as well as Understanding Mono C# Compiler book.