Should I raise an exception when getting the size or the position of a file? - windows

Question

I'm trying to make a class in Delphi that handles files. I have a property that returns the size of the file and another one that returns the position of the file. I don't know if any error can happen with these calls. Should I raise an exception?
My code is:
function TFile.GetSize: Int64;
var
FileSizeHi, FileSizeLo: Cardinal;
begin
FileSizeLo := GetFileSize(FHandle, #FileSizeHi);
if (FileSizeLo = INVALID_FILE_SIZE) and (GetLastError = NO_ERROR) then
Result := $FFFFFFFF
else
Result := FileSizeLo or Int64(FileSizeHi) shl 32;
end;
function TFile.GetPosition: Int64;
var
FilePosHi, FilePosLo: Cardinal;
begin
FilePosHi := 0;
FilePosLo := 0;
FilePosLo := SetFilePointer(FHandle, FilePosLo, #FilePosHi, FILE_CURRENT);
if (FilePosLo = INVALID_SET_FILE_POINTER) and (GetLastError = NO_ERROR) then
Result := $FFFFFFFF
else
Result := FilePosLo or Int64(FilePosHi) shl 32;
end;
I don't know what error could happen when I call GetFileSize or SetFilePointer (without moving the file pointer).

Answer 1

Yes, errors can happen with those functions, so I would recommend raising an exception, otherwise the caller doesn't know if it has received an invalid value or not, as $FFFFFFFF is a valid size/position for 64bit values. Perhaps you meant to use -1 ($FFFFFFFFFFFFFFFF) instead?
However, whether you raise an exception or not, your GetLastError() check is wrong. It needs to use <> instead of =. When the file function returns $FFFFFFFF for the low value, GetLastError() will return 0 when the low value really is $FFFFFFFF, otherwise GetLastError() will return non-zero when the low/high values are invalid.
Try this:
function TFile.GetSize: Int64;
var
FileSizeHi, FileSizeLo: DWORD;
begin
FileSizeLo := GetFileSize(FHandle, #FileSizeHi);
if (FileSizeLo = INVALID_FILE_SIZE) and (GetLastError <> NO_ERROR) then
RaiseLastOSError // or: Result := -1
else
Result := FileSizeLo or (Int64(FileSizeHi) shl 32);
end;
function TFile.GetPosition: Int64;
var
FilePosHi, FilePosLo: DWORD;
begin
FilePosHi := 0;
FilePosLo := 0;
FilePosLo := SetFilePointer(FHandle, FilePosLo, #FilePosHi, FILE_CURRENT);
if (FilePosLo = INVALID_SET_FILE_POINTER) and (GetLastError <> NO_ERROR) then
RaiseLastOSError // or: Result := -1
else
Result := FilePosLo or (Int64(FilePosHi) shl 32);
end;
On a side note, consider using GetFileSizeEx() and SetFilePointerEx() instead, as they operate on 64bit values without breaking them up into low/high parts.