Inno Setup - Check if file exist in destination or else if doesn't abort the installation

Question

I need my installer to check if a file exists in the destination location, and if is not there, then the installation aborts. My project is a update patch, so I want the installer to avoid installing the update files if the main exe of the application is not in the destination. How can I do this?

Can someone give an example of code to check file version through the Windows registry?

[Files]
Source C:\filename.exe; DestDir {app}; Flags: ignoreversion; BeforeInstall: CheckForFile;

[code]

procedure CheckForFile(): Boolean;
begin
  if (FileExists('c:\somefile.exe')) then
  begin
    MsgBox('File exists, install continues', mbInformation, MB_OK);
    Result := True;
  end
  else
  begin
    MsgBox('File does not exist, install stops', mbCriticalError, MB_OK);
    Result := False;
  end;
end;

Answer 1

Just don't let the user proceed until they pick the correct folder.

function NextButtonClick(PageId: Integer): Boolean;
begin
    Result := True;
    if (PageId = wpSelectDir) and not FileExists(ExpandConstant('{app}\yourapp.exe')) then begin
        MsgBox('YourApp does not seem to be installed in that folder.  Please select the correct folder.', mbError, MB_OK);
        Result := False;
        exit;
    end;
end;

Of course, it's also a good idea to try to automatically pick the correct folder for them, eg. by retrieving the correct location out of the registry.