File Associations in Windows Regsitry

See [1] for the description of HKLM, HKCU, and HKCR. According to [2], HKCR provides a merged view of HKCU and HKLM. For ProgID, see [3]. For file extension, see [5].

File extensions config examples (see [7] for valid keys/entries):

for .rar:

• HKLM/SOFTWARE/Classes/.rar has a key "(Default)" with ProgID "7-Zip.rar"
• HKCU/Software/Microsoft/Windows/CurrentVersion/Explorer/FileExts/.rar
  • child: OpenWithProgids has a key named "7-Zip.rar" (appear in "open with" menu)

for .txt:

• HKLM/SOFTWARE/Classes/.txt (apply to all users)
  • keys: (Default)=txtfile; Content Type=text/plain (txtfile is ProgID)
  • child: OpenWithProgids (appear in "open with" menu)
  • child: ShellNew
• HKCU/SOFTWARE/Classes/.txt (apply to the interactive user)
  • Does not exist in my case
• HKCU/Software/Microsoft/Windows/CurrentVersion/Explorer/FileExts/.txt (apply to the interactive user)
  • child: OpenWithProgids has a ProgID "txtfile"
  • child: UserChoice has a key named "Progid" with value "Applications/notepad++.exe" (!important)

Applications with ProgID:

• HKLM/SOFTWARE/Classes/7-Zip.rar/shell/open/command (7-Zip.rar is ProgID)
• HKLM/SOFTWARE/Classes/txtfile/shell/open/command: the value of "(Default)" is the command used to open files.

How a file extension (e.g. .txt) is mapped to a file type?

• "UserChoice" takes the first precedence. If it is not set, key "(Default)" stores the ProgID which can be found under [HKCU|HKLM}/SOFTWARE/Classes/.

For gvim.exe, you should have HKLM/SOFTWARE/Classes/Applications/gvim.exe/shell:

• edit/command[(Default)] = command
• open/command[(Default)] = command
  Notes: "gvim.exe" is ProgID, see [3] for more detail.

How to change the default app when a file is double-clicked? (Take .c as an example)

• Solution 1 (use entries for Explorer)
1. For HKCU/Software/Microsoft/Windows/CurrentVersion/Explorer/FileExts/.c/UserChoice, set the value of key "UserChoice" to "Applications/gvim.exe".
2. For HKLM/SOFTWARE/Classes/Applications/gvim.exe/shell/edit/command, set the value of key "(Default)" to '"C:\Program Files (x86)\Vim\vim73\gvim.exe" -p --remote-tab-silent "%1" "%*"'
3. Open "Control Panel\Programs\Default Programs\Set Associations", you will see .txt has been associated with gvim.exe.
• Solution 2
  UserChoice should not be set in this solution (you can delete it first).
1. make sure HKLM/SOFTWARE/Classes/.c has the key "(Default)" set to "cfile".
2. For HKLM/SOFTWARE/Classes/cfile/shell/open/command, set "(Default)" to the gvim command above.

References

[1] http://msdn.microsoft.com/en-us/library/windows/desktop/ms724475%28v=vs.85%29.aspx
[2] http://msdn.microsoft.com/en-us/library/windows/desktop/ms724498%28v=vs.85%29.aspx
[3] http://msdn.microsoft.com/en-us/library/windows/desktop/dd542719%28v=vs.85%29.aspx
[4] http://msdn.microsoft.com/en-us/library/windows/desktop/ms690440%28v=vs.85%29.aspx
[5] http://msdn.microsoft.com/en-us/library/windows/desktop/ms678415%28v=vs.85%29.aspx
[6] http://msdn.microsoft.com/en-us/library/windows/desktop/hh127445%28v=vs.85%29.aspx
[7] http://msdn.microsoft.com/en-us/library/windows/desktop/cc144148%28v=vs.85%29.aspx#fa_optional_keys_attributes

Sample reg file

Be careful, it will overwrite the current value!!!
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Windows Registry Editor Version 5.00 

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.c] 
@="cfile" 

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\cfile\shell\open\command] 
@="\"C:\\Program Files (x86)\\Vim\\vim73\\gvim.exe\" -p --remote-tab-silent \"%1\" \"%*\"" 

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Note: the value must be a string put in double quotations!