Why are .fsx scripts executed when a compiled program is started?

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Summary

When an .fsx script is included in the <Compile> tag of an .fsproj file, it is executed alongside the compiled program during dotnet run. This behavior is due to F# Interactive (FSI) integration in the build process, which treats .fsx files as executable scripts.

Root Cause

  • FSI Integration: .fsx files are designed for F# Interactive and are executed when compiled into the project.
  • Build Configuration: Including .fsx in <Compile> ensures it is part of the build output, triggering execution during runtime.

Why This Happens in Real Systems

  • Design Intent: .fsx files are meant for interactive scripting, and compiling them preserves their executable nature.
  • Build System Behavior: The .NET build system does not differentiate between .fs and .fsx files when included in <Compile>, treating both as code to execute.

Real-World Impact

  • Unexpected Output: Scripts like printfn "Hello from Script!" run alongside the main program, cluttering output.
  • Performance Overhead: Unnecessary script execution can slow down application startup.
  • Debugging Complexity: Identifying the source of unexpected behavior becomes harder.

Example or Code (if necessary and relevant)



  
    Exe
    net8.0
  
  
    
     
  

// Development.fsx
printfn "Hello from Script!"

How Senior Engineers Fix It

  • Exclude from Compile: Move .fsx files to <Content> or <None> to prevent execution:
  • Conditional Compilation: Use #if INTERACTIVE to limit script execution to FSI only: 
    #if INTERACTIVE
printfn "Hello from Script!"
#endif

Why Juniors Miss It

  • Lack of FSI Knowledge: Juniors may not understand the difference between .fs and .fsx files.
  • Overlooking Build Configuration: Assuming <Compile> is harmless for scripts without realizing its impact.
  • Minimal Documentation: Limited guidance on .fsx behavior in .NET project files.

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