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Editorial Team
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Asked: 2026-06-07T08:55:10+00:00

I’ve installed Mono successfully with the mac installer (which includes F#), and have been

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I’ve installed Mono successfully with the mac installer (which includes F#), and have been playing around with the interactive shell. I can also compile F# source files just fine to produce a .NET EXE, but when I try to run it with Mono, I get the error:

Unhandled Exception: System.TypeLoadException: Could not load type '<StartupCode$HelloWorld>.$HelloWorld' from assembly 'HelloWorld, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
[ERROR] FATAL UNHANDLED EXCEPTION: System.TypeLoadException: Could not load type '<StartupCode$HelloWorld>.$HelloWorld' from assembly 'HelloWorld, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.

The contents of the file — HelloWorld.fs — is simply

printfn "Hello world"

I’ve also read this question, but the solution does not fix my problem. Any ideas?

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1 Answer

  1. Editorial Team

    Turns out, I had 2 different mono versions installed in two separate locations — 2.10.6 in /opt/local/lib/mono and 2.10.9 in /Library/Frameworks/Mono.framework. The fsi command was symlink to the fsi.exe that shipped with the latter, while the mono command itself was another symlink pointing to the former.

    Lesson learned: Make sure you don’t have two versions of the same software installed

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