Why CurrentMember Can Be Used Only On Hierarchies

I used to wonder why the MDX function CurrentMember applies only to hierarchies and not to levels.  For example, the Members function can be used on either a hierarchy or a level, and returns the appropriate set of members.  I thought it would make sense if CurrentMember worked in a similar way.

Today while reading Chapter 10 - MDX Concepts of the book Microsoft SQL Server 2008 Analysis Services Unleashed, it suddenly hit me.  I realised why CurrentMember should only be applied to hierarchies and not to levels.

The book was explaining the concept of Execution Context and how SSAS works out the current coordinates for a query.  The SSAS engine starts by setting the current coordinates to the default member of each attribute hierarchy.  In most cases this would be the All member.  Then it takes the members that have been specified in the query and overwrites the appropriate attributes.  However, the coordinates for those attributes not specified in the query will remain at their default member.  Therefore in most cases, these attributes' CurrentMember is still the All member.


So that's why the CurrentMember function can be applied only to hierarchies and not to a specific level: because it needs to be able to return any member from the whole hierarchy, including the All member.  Applying the function to a specific level could return invalid results.


Rethinking about this, it all seems be a bit obvious now, but I guess it never clicked for me before.  So I am happy to share it in case you have wondered about the same thing.