Hi,
You can access to a closed document database using the Database.ReadDdwg() method.
You can find a sample in the AutoCAD .NET Developer's Help >Develop Applications with VB.NET and C# > Handle Errors > Trap Runtime Errors > Use Try Statements
http://docs.autodesk.com/ACD/2010/ENU/AutoCAD%20.NET%20Developer%27s%20Guide/index.html