Hi,

Partial classes would be useful if you did want to compile your custom methods into dbPortal, but it is not what I recommend, it is better not to mingle your custom code with the core code.

dbYourFeature is the correct solution. It does not need to and should not extend or have anything to do with dbPortal. Ideally you would write multiple versions of dbYourFeature to support the multiple databases if you were planning to sell or share your custom feature but this is not needed to use it yourself.
You would only need to reference the ADO.NET driver layer, like for MySQL your dbFeatureName would reference MySql.Data, for MS SQL the driver is built into the.NET framework (System.Data.SqlClient) but you might want to copy the SqlHelper class from the mojoportal data layer as this helper class simplifies the code you write quite a bit.
The MySQL Connector has a similar MySqlHelper included with it. For PostgreSQL you might want to borrow the pgsqlHelper class, it may later become part of the npgsql project and be included.

You can write complete features that are entirely in separate assemblies and plug into mojoportal without modifying mojoportal code. Your web ui would need to reference mojoPortal.Web and the Module control which is the entry point of your feature would need to inherit from SiteModuleControl. If you look in the VS Solution there is a SampleExternalModule project that illustrates this. The main trick is a post build action that copies the needed files into the appropriate places beneath the Web folder. If you right click the project node in VS solution explorer and got to properties you can find the post build actions.

I will be writing a tutorial on implementing custom features as external projects very soon.

Best Regards,

Joe