Question

I want to let the user select an association (open with) for an currently unregistered file extension.

Currently I’m telling the API to open the file by using ShellExecute and it returns an ERROR_NO_ASSOCIATION error code.

Is there a way to tell the API that it should let the user select a new association?

Answer 1

I use

procedure ShellOpenAs(const AFileName: string; AHandle: HWND);
begin
  ShellExecute(AHandle, 'open', PChar('rundll32.exe'), PChar('shell32.dll,OpenAs_RunDLL ' + AFileName), nil, SW_SHOWNORMAL);
end;

Edit (inspired by David’s comment and https://stackoverflow.com/a/13229516/1431618):
One can omit ShellExecute and RunDll32 by calling OpenAs_RunDLL directly:

procedure OpenAs_RunDLL(hwnd: HWND; hinst: HINST; lpszCmdLine: LPSTR; nCmdShow: Integer); stdcall; external shell32;

procedure ShellOpenAs(AHandle: HWND; const AFileName: string);
begin
  OpenAs_RunDLL(AHandle, HInstance, PChar(AFileName), SW_SHOWNORMAL);
end;

There is also SHOpenWithDialog on Windows Vista and later. (I find it interesting that Microsoft wrote a RunDLL compatible entry point but until Vista didn’t bother to provide a regular API function.)