The Upgrade Toolkit objects that shipped with Microsoft Dynamics NAV 2013 have a good example of code that works directly with SQL Server through the .NET Framework library. On the product media, navigate to this location:
..\UpgradeToolKit\Data Conversion Tools\601\
Import the Upgrade 601700.2.fob file. This contains codeunit 104050 Upgrade – SQL Mgt.:
and codeunit 104049 Upgrade Dimension entry – SQL:
The same code structure should be applicable to Microsoft Dynamics NAV 2009 R2, though only for the RoleTailored client.
I hope this helps you create codeunits that connect to SQL Server through .NET Interop.