Expression-based Connection Strings Difficult To Manage

Expression-based connection strings are great but they are specific to each report which makes them difficult to maintain. Shared expression-based connection strings are not supported.



I'm looking for a way around this. I'm hoping to write a custom data extension, and in the custom data extension, do all the "dynamic" logic that sets the connection string (which you normally would be doing in your expression-based connection string).



I know the above can be done. What I don't know is how, inside a custom data extension, to get the value of expressions like User!UserID? (I want the connection to run under stored credentials, but I want to customize the connection string and add a property to it based on the UserID who is logged into Report Manager.)