Delphi 2010 - Is File in Use?

Question

I have a function which works ok to check if a local file is in use. However if I map a network drive and try to check if a file from the mapped drive is in use then the result of the function is always false. I need to wait before a large file is being copied to the mapped drive and after completion I rename the file. If the file in not in use then i start performing various actions else i wait another minute and check again. How can I modify the function below in order to work with mapped drive files that are constantly copied?

Thank you

function IsFileInUse(FileName: TFileName): Boolean;
var
  HFileRes: HFILE;
begin
  Result := False;
  if not FileExists(FileName) then
  begin
  showmessage('Fisierul "'+Filename+'" nu exista!');
  Exit;
  end
  else
  begin
  HFileRes := CreateFile(PChar(FileName),
                         GENERIC_READ or GENERIC_WRITE,
                         0,
                         nil,
                         OPEN_EXISTING,
                         FILE_ATTRIBUTE_NORMAL,
                         0);
  Result := (HFileRes = INVALID_HANDLE_VALUE);
  if not Result then
    CloseHandle(HFileRes);
  end;
end;

Answer 1

What you are claiming is that CreateFile succeeds in opening a file in exclusive mode whilst another party is writing to the file. Possible explanations:

1. You have made a mistake with the file name and the file you are opening is not the one in use.
2. The other party is writing to the file without having locked it. In other words it opened the file with a share mode that allowed other parties to read and write. This would be quite unusual. If this is the case then you need to fix the other process.
3. The remote file server is broken and fails to respect locks. I'd regard this as quite unlikely.

I think the final option can be rejected immediately. Option 2 seems rather unlikely. Which leaves option 1. You are able to lock the file because it is not locked.

I'd also comment that the function is spurious. You can remove it. Simply attempt whatever operation you need to perform. If that operation fails due to a sharing violation you know that the file was locked. Consider also the race condition in any code using that function. The fact that a file is unlocked now does not prevent another party locking the file before you can do anything with it.