Delphi Determine filesize in real time

Question

Is it possible in Delphi to determine the size of a file as it is being copied? I get a notification when a file is first copied to a folder, but need to wait until the copy is complete before I can process the file.

I've used JclFileUtils.GetSizeOfFile(Filename) but that gives me the 'expected' file size, not the current filesize.

Regards, Pieter

Answer 1

Prompted by the first answer I decided to give up on trying to determine when a file copy has completed. Instead I found that using TFileStream gave me a reliable indication whether a file is in use or not.

function IsFileInUse(Filename: string; var ResultMessage: string): boolean;
var
  Stream: TFileStream;
begin
  Result := True;
  ResultMessage := '';
  try
    Stream := TFileStream.Create(Filename, fmOpenRead or fmShareDenyWrite);
    try
      Result := False;
    finally
      FreeAndNil(Stream);
    end;

  Except on E: Exception do
    ResultMessage := 'IsFileInUse: ' + E.Message
  end;
end;

In this way I can keep on checking until the file is not in use anymore before attempting to process it.

Answer 2

It depends on the technique that is used by the copying function. Most copy-methods will allocate the disk space first before they start to copy a file. Thus, if you want to copy a file of 4 GB, the system starts by creating a file with random data for 4 GB in total. (Which is done lightning-fast, btw.) It then copies the data itself, but the file size is already what you expect. This has as advantage that the sysmen can check if there's enough disk space available to actually copy the data.

If you write your own file copy function then you can have total control over how it does this. Else, you're limited to whatever the chosen copy-method offers you. So, how do you copy a file?

Answer 3

If you have control over the file copy process, it is easiest to have the copy routine create the file using a temporary filename, and when done, rename it to correct filename. That way, you can use Windows folder monitoring to watch for the renaming (JCL contains a component to help with this, not sure about the name from here). When your code gets triggered you are sure the other side has finished writing the file. A simple trick I used was to have the copying process create new files with a '$$$' extension. My code still got triggered for those but I ignored them until they were renamed to their proper filename.

Hope this helps.