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

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.

Answer 2

Another solution would be the InitializeSetup():
Credit: Manfred
[code]
function InitializeSetup(): Boolean;
begin
if (FileExists(ExpandConstant('{pf}\{#MyAppName}\somefile.exe'))) then
begin
MsgBox('Installation validated', mbInformation, MB_OK);
Result := True;
end
else
begin
MsgBox('Abort installation', mbCriticalError, MB_OK);
Result := False;
end;
end;