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var
 stream: TFileStream;
begin
 stream := TFileStream.Create('foo.txt', fmOpenRead);
 setlength(deinstring, stream.Size);
 stream.Read(pointer(deinstring)^, stream.Size);
 stream.free;
end;                    

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procedure TForm1.Button1Click(Sender: TObject);
var
  f : TextFile;
  s : String;
begin
  AssignFile(f,'C:\Test.txt');
  Reset(f);
  ReadLn(f,s);
  CloseFile(f);

  Edit1.Text := s;
end;