Since most of the parts of .NET is open-sourced, ILSpy also works with Linux and Mac.
From project GitHub page:
Decompiler Frontends
Aside from the WPF UI ILSpy (downloadable via Releases, see also
plugins), the following other frontends are available:
- Visual Studio 2017/2019 extension marketplace
- Visual Studio Code Extension repository | marketplace
- ICSharpCode.Decompiler NuGet for your own projects
- Linux/Mac/Windows ILSpy UI based on Avalonia – check out https://github.com/icsharpcode/AvaloniaILSpy
- Linux/Mac/Windows command line client – check out ICSharpCode.Decompiler.Console in this repository
- Linux/Mac/Windows PowerShell cmdlets in this repository
Since VS Code works with Linux and Mac, that front end should work in those operating systems too. Command line client should also work.
In addition, JetBrains has a new IDE for .NET, Rider. It works in Linux and Mac. Rider has a built-in decompiler. Unfortunately, this built-in compiler, dotpeek, was only available for Windows. But according to comments, dotpeek works in Mac as of 2023 Summer.
Another choice is to use mono decompiler, a tool from monoproject. Try to use it. Note that this is an add-in in MonoDevelop.