[]
The base directory that contains workbook.
If the value is not null (Nothing),
Name is the file name of workbook.
If the value is empty, the path is the root directory.
public string BaseDirectory { get; }Caution: Combine(string, string)
has different strategy for Empty and
null (Nothing).
Check this value with IsNullOrEmpty(string)
(or use = Nothing in Visual Basic)
before passing it to Combine(string, string).