I'm not an expert in PowerShell, so I do not know what the syntax \{program_files(x86)} means. But your command does not work for me either.

If I use the path explicitly, it works:

.\RegAsm "C:\Program Files (x86)\WinSCP\WinSCPnet.dll" /codebase /tlb

Tried several times, next step may be to try on a 32-bit machine to see whether it too doesn't like the "unknown option" - is this dll suitable for x86 / 32-bit only, or is there a work-around?

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PS C:\windows\microsoft.net\framework\v4.0.30319> .\RegAsm \{program_files(x86)}\WinSCP\WinSCPnet.dll /codebase /tlb
Microsoft .NET Framework Assembly Registration Utility version 4.6.1055.0
for Microsoft .NET Framework version 4.6.1055.0
Copyright (C) Microsoft Corporation. All rights reserved.

RegAsm : error RA0000 : Unknown option: /encodedCommand
PS C:\windows\microsoft.net\framework\v4.0.30319>

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Thanks all :)