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Editorial Team
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Asked: 2026-06-15T15:42:07+00:00

Context For a given file extension, I need to find out the ProgID(?) of

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Context

For a given file extension, I need to find out the ProgID(?) of the file association (e.g. Word.Document for .docx). I need his (a) to recognize specific associated software (e.g. Adobe versus Nuance) and handle accordingly before they can open a document, and (b) to make Outlook display the correct icon for virtual files (message class IPM.Document.[ProgID]).

The current solution

I simply read out the (Default) entry of the registry key at HKEY_CURRENT_ROOT\[extension]

The problem

On some machines, for some file types, the (Default) entry in HKCR is empty., However, the Windows Explorer (shell) shows the appropriate icon, and definitely has a working file associations when opening the file.

Clearly, I am not using the right approach to retrieving the file association. Is there a ASSOCSTR I am missing that I can pass to AssocQueryString that will return this information, or any other call or standard documentation on file associations on MSDN that I am overlooking?

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1 Answer

  1. Editorial Team

    Since Windows Vista (I think) there has been the ability for the user to override the normal HKCR file association system, via the key

    HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\
    .extension\UserChoice\

    You need to check this key for the ProgId value and use that if it exists, and treat HKCR\extension as a default fallback only.

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