I always thought that this was a reliable way to determine which program is used to open a file with a certain extension when you double click it in Windows Explorer.

C:\>assoc .txt

C:\>ftype txtfile
txtfile=%SystemRoot%\system32\NOTEPAD.EXE %1

The above is my actual result. But it is UltraEdit that opens my text files.

Is this method not so reliable after all? Where can I see (and change), with a command or in the registry, which program opens my text files?

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    Check also reg query "hkcr\.txt" /S to see if there are OpenWithList or OpenWithProgIDs registered programs (and what is (default) ). – LotPings Jun 14 at 10:20
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    Command-line association and Explorer association may differ. Check HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.txt\UserChoice. – Akina Jun 14 at 10:28
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    reg query HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.txt\OpenWithList – JosefZ Jun 14 at 13:11

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