Disclaimer: The information below is not to be considered the absolute truth. It is simply what worked for me on Windows in regards to getting my Mono code to run on Linux and Mac.

0. Prerequisites

Visual Studio 2017
- When installing add: Clang and the VS SDK.
Mono (I use version 5.16.0)
Knowledge of terminal commands and computers.

1. Downloading Target SDK

Before you start you need to download Mono SDK packages for the platforms you will be targeting. Your version of Mono, which you can check using mono --version, must match the package version. Pick one here and add them using:

mkbundle --fetch-target $target --target-server http://download.mono-project.com/runtimes/raw/

where $target is the name of the target you want to add. On Mono 5.16.0 the targets I have installed are:

mono-5.16.0-osx-10.7-x64 and mono-5.16.0-debian-9-x64

2. Packaging

After your application has been built you will need to statically link it in order for the code to run without Mono installed on the user's machine.

To do so run the following command, where:

$OutputFileName - Is the name of the output file. No extension is needed.

$ExeName - The name of the exe to package.

$Target - The name of the target you will be compiling for.

mkbundle -o $OutputFileName $ExeName --deps --static --cross $Target

This command should work for most purposes, but optionally you can add a machine.config to ensure environment similarity by adding the following argument to the command above.

--machine-config $config

The config is usually found in %windir%\Microsoft.NET\Framework64\$version\config\machine.config for 64-bit systems or %windir%\Microsoft.NET\Framework\$version\config\machine.config for 32-bit systems.

For example: C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Config\machine.config.

Note: Some .NET versions use the same CLR, so for instance version v3.0 and v3.5 will not contain a machine.config - they will use the one present in version 2.0.50727.

Also optionally you can add a specific implementation of the standard library by specifying -L $MonoPath where $MonoPathis the path where the framework is installed on your computer and find your .Net version, for me this was C:\Program Files\Mono\lib\mono\4.5

Note: Its better not to do this, but it might solve your issue.

Tip: You can easily find your config by running the [System.Runtime.InteropServices.RuntimeEnvironment]::SystemConfigurationFile command in PowerShell.

Tip: After packing you can remove non-native DLLs (managed). They are no longer required and will be loaded from the package you've created, this is important to remember, as you need to repackage when updating them.

3. Platform Specific Fixes

Linux
- An error similar to DLLNotFound: lib.c can be fixed by including the System.dll.config file from this repo with your application.
- Terminal applications will not open the terminal unless opened from the terminal. I have no solution for this other than invoking Linux stuff to open a console window yourself.
Mac
- File might open as a text file when double-clicked. In cases like this running chmod +x {FileName} worked. You might want to do the same for the mkbundle result as well.
- Sometimes the dllmap file will not work when the application is double-clicked but will work when launched from the console from the current folder, resulting in DLLNotFound exceptions. It will however not work if launched from another folder. The fix I use for this is to include a .command file which will cd to the current folder and launch the application. I've included a template for such a script in this repo under the name MacRunScriptTemplate.command. Don't forget Windows line endings as those will break the script on Mac.

4. Notes

Paths in .dll.config files on map should be relative. ex. './Libraries' instead of '/Libraries'.
Some libraries like ServiceStack.Text will not work when packaged for some reason, this can happen when libraries invoke unmanaged code or are generally not path agnostic. I have no solution for this, look for alternatives.
When choosing the MonoPath in step 2 look out for folders ending in -api like 4.7-api. They do not work as expected.
If you receive an error when launching your application like Unexpected character ")" this means you've packaged for the wrong platform. Check your --cross argument from step 2. Things like x86 and x64 can play a factor.
- The Linux (and Windows with cygwin) command file {path} can give you insight into what you've just packaged.
Sometimes your .dll.config file correctly points to the platform library but execution will still claim it cannot find it. This happens when your unmanaged library requires another library which is missing. To debug this use the MONO_LOG_LEVEL=debug mono $Exe.exe command where $Exe is the compiled non-bundled executable, ldd on Linux, or otool on Mac. To resolve these issues you need to include the missing libraries and either change the LD_LIBRARY_PATH on Linux or the DYLD_LIBRARY_PATH on Mac using a script such as MonoKickstart (linked below) or you can dlopen them manually. For examples go here. Usually that does the trick on Linux, but on Mac you might need to use the otool and the install_name_tool tools. With otool -L {lib}.dylib you can see where it expects certain libraries to be and with install_name_tool -change {oldPath} {newPath} {lib}.dylib you can change that path. The paths are relative to the executable and not the library, and after you change them you can redistribute the modified library.
You can use ldconfig -p | grep <library name> to find where a library is installed (and if it is at all) on Linux.