How do I query "Size on disk" file information?

Question

I want to reproduce behaviour exhibited in the Windows Explorer -> Properties dialog -> General property page for any given file. Specifically I want to reproduce the exact value of the "Size on disk" field.

Answer 1

As others have said, you need to use GetFileInformationByHandleEx, but it looks like you need to use FILE_STANDARD_INFO or FILE_ID_BOTH_DIR_INFO. The information you want is returned in the AllocationSize member of each, but the second is for directory handles, to list files within instead of the directory itself (note: not recursive, just top-level). To make it easier, FILE_STANDARD_INFO has a Directory boolean, so call it first if you're not sure. According to the documentation for FILE_ID_BOTH_DIR_INFO,

AllocationSize Contains the value that specifies how much space is allocated for the file, in bytes. This value is usually a multiple of the sector or cluster size of the underlying physical device.

This seems to give you the Size on Disk information.

I haven't found a Delphi translation of the FILE_ID_BOTH_DIR_INFO structure. The difficulty would seem to be the final member, WCHAR FileName[1], which is described as:

FileName[1]
Contains the first character of the file name string. This is followed in memory by the remainder of the string.

I'm not sure how this would be handled in Delphi.