Shall the returned data stream report a proper length with its IDataStream.Length property? If set to false, the stream may still report a proper length (for example, if doing so would not incur any significant overhead, see IFile.IsLocal). This parameter should be set to true only if the calling code actually requires proper length values (for example, to read a data block at the end of a file) and should be left false when length values are nice-to-have only (for example, to compute a progress value for some background operation).