Attach a database file on connect to a local SQL Server Express instance using SQL Native Client ODBC

Connection string

This SQL Native Client 9.0 ODBC Driver connection string can be used for connections to SQL Server 2005.

Driver={SQL Native Client};Server=.\SQLExpress;AttachDbFilename=c:\mydbfile.mdf;

Why is the Database parameter needed? If the named database have already been attached, SQL Server does not reattach it. It uses the attached database as the default for the connection.