Issue with cs-script Compilation in Unity Standalone Build (.NET Framework 4.8)

[pavelkux]

Hello,
I realize my situation might be an edge case, as not many people are using the .NET Framework branch—let alone within Unity. If you decide this isn’t worth pursuing, I would completely understand.

That said, I was hoping you might be able to shed some light on the issue I'm facing:

• I’m developing a game in Unity.
• It includes a business layer written in .NET Framework 4.8.
• The game uses a library with cs-script for running player-created scripts.
• Everything works perfectly in the Unity Editor, and it also worked previously outside Unity.
• However, the issue arises when I build a standalone version of the game from Unity.
• It crashes when it attempts to compile scripts on the fly.

The exception is:

Error running D:\GamePath\Tests\mono\mini\mono.exe: System cannot find the file specified.
Interestingly, there’s no mini folder (and never was). The actual build is located at D:\GamePath\Tests\Game\Build
It seems the issue stems from Unity Editor using its own Mono runtime to compile the scripts successfully, but in the standalone build, the stripped-down version of Mono provided by Unity cannot compile code dynamically.

One potential fix I considered is distributing Mono alongside the game (or requiring it as a prerequisite) and manually setting the path to mono.exe. Unfortunately, I haven’t been able to find a way to configure this path in cs-script.

Do you have any suggestions for resolving this? Or perhaps an alternative approach I could try?

Thank you in advance!

[oleg-shilo]

Unfortunately.... you are right.
This is teh problem with Unity is that on the target environment the runtime is lacking CodeDomProvider (System.CodeDom.Compiler) implementations.

There is a way to do that though. CS-Script allows bringing any compiler with an alternative compiler assembly supplied with -provider switch in CLI. Or options.altCompiler in the class library.
The implementation of alternative compiler assembly exists. You may try to use it as is or even adjust it to your needs: https://github.com/oleg-shilo/cs-script.net-framework/tree/932d96d1a06b308cb565b1785dc3dccfcc26036d/Source/CSSRoslynProvider

However, it is a difficult question if the effort worth the outcome. Only you can answer it.

[pavelkux]

Hello Oleg,
Thank you for your response - I really appreciate it!

I’ve gone through the links you shared and dug a bit deeper into the issue. Since all my scripts are compatible with C# 5, I’m considering packaging the Mono distribution (including mono.exe, mcs.exe, and all the required libraries) with the game and then using altCompiler to target those files.

I plan to test this approach soon, but before I proceed, I wanted to check if you think this is a viable solution—or if you already know of any limitations or issues that might prevent it from working.

Thanks again for your help!

[oleg-shilo]

This is a very right question...
I would break it down further:

1. Is it possible?
   You can easily test it without an overinvestment.
   Deploy mono to your runtime environment and check if you can compile a simple hello-world assembly from C# code.
   If you can, then you totally can integrate it into CS-Script.

2. If it is possible, is it worth the effort?
   This is the question that I cannot answer. Both effort and benefit are hard to estimate.

If the answer to question #1 is "No", then the #2 is irrelevant

As for #2 you have the option to completely avoid any integration and simply build an assembly at runtime (mono.exe + script => assembly). Load this assembly and then use reflection to invoke the method from this assembly. It's a very simple, robust approach, but it is only justified if your assembly has a fixed, simple interface and fixed dependencies. And if I were you, I would love to just get away with this approach

However, if you need to be flexible, then you need a helper for managing dependencies, loading, reflection (invoking) and caching. And this option will require you either to create such an adaptor by yourself or to use CS-Script for that.

As for complexity of integrating CS-Script, it's not too difficult, but it may take some time since it's a new domain for you.
Another point is that you may start feeling that running mono compiler programmatically is a bit hacky. But.. it's actually not. When I was working on alternativeCompiler concept, I reverse-engineered MS CodeDomProvider and found that it is nothing else but a slightly overengineered runner for csc.exe (C# CLI compiler). :)

[pavelkux]

Thank you very much for your reply!

If you're currently too busy to engage further, please don’t hesitate to let me know — I would completely understand.

As someone who’s relatively new to Mono and the broader topic of dynamic compilation in .NET (aside from having used CS-Script extensively for years), I have a few more questions. Some of them may seem basic, but I'm doing my best to fully understand the situation and the available options.

1. I'm a bit surprised that such a common use case — C# scripting in Unity using Mono — doesn't already have a straightforward documented workaround. Is there something I might be missing here?

2. Is it possible to use Mono.CSharp as the compiler backend with CS-Script? https://www.mono-project.com/docs/about-mono/languages/csharp/

3. Another idea I considered: using the System.dll from Unity Editor assemblies (which includes CSharpCodeProvider) and then, via reflection, loading the assembly and accessing the Microsoft.CSharp.CSharpCodeProvider type to cast it to ICodeCompiler. While the type is found, the cast fails. It seems like it is not having that interface?

4. If I decide to switch to Roslyn, is there anything I should be aware of that might break? I'm currently using a fairly old version of CS-Script (from 2016, before Roslyn was supported), and I’ve made several custom changes that I’d need to re-apply if I update. Do you think switching to Roslyn would fully resolve the issue, or are there known limitations or edge cases I should consider?

Thanks again for your time and guidance - it's genuinely appreciated.

[pavelkux]

It actually seems that I was able to get everything working with Roslyn and your CSSRoslynProvider, both in .NET Framework and under Mono in Unity - within the Unity Editor as well as in standalone builds.

To be honest, I can hardly believe it myself, so I hope I'm not mistaken. I’ll run more tests to confirm everything is working as expected.

[pavelkux]

Yes, it works. I set CSScript.GlobalSettings.UseAlternativeCompiler and CSScript.GlobalSettings.RoslynDir and distribute Roslyn with the game and all works great without any other modifications.
Thank you for your help Oleg!

[oleg-shilo]

Glad you sorted it all out