Delphi: deny access to file for other processes - delphi

Question

How can I deny access (only to write) to a file for other processes? I will read\write a file all time.
I use
FileOpen('c:\1.txt', fmOpenReadWrite or fmShareDenyWrite)
but after (starting to load the file to StringList) I get error
Cannot open file C:\1.txt. The process cannot access the file because it is being used by other process."
Only I open the file.

Answer 1

Here, the error message is actually slightly misleading. The reason you can't load into the stringlist is because you already opened the file in read/write.
if you check the implementation of TStrings.LoadfromFile:
procedure TStrings.LoadFromFile(const FileName: string);
var
Stream: TStream;
begin
Stream := TFileStream.Create(FileName, fmOpenRead or fmShareDenyWrite);
try
LoadFromStream(Stream);
finally
Stream.Free;
end;
end;
You see that it tries to open the file with a "DenyWrite" condition, but you already opened the file in write mode. That is the reason why it fails.
You can work around that by using LoadFromStream instead.
procedure TForm1.Button1Click(Sender: TObject);
var
Stream: TStream;
begin
Stream := TFileStream.Create(FileName, fmOpenRead or fmShareDenyNone);
try
Stringlist.LoadFromStream(Stream);
finally
Stream.Free;
end;
end;
Note that you will need to use fmShareDenyNone for this to work in that situation. Then again, you could probably reuse the Read/Write handle you got from your OpenFile, probably do something like this:
procedure TForm1.Button1Click(Sender: TObject);
var
Stream: TStream;
iPosition : Int64;
begin
Stream := THandleStream.Create(FHandle); //FHandle is the read/write handle returned by OpenFile
try
iPosition := Stream.Position;
Stream.Seek(0, soFromBeginning);
Stringlist.LoadFromStream(Stream);
Stream.Position := iPosition;
//Restore stream position.
finally
Stream.Free;
end;
end;
But be advised that these approach might have a few "gotchas" I'm unaware of.