What is the difference between “using System.Data;” and directly
adding the reference from “Add References Dialog Box”?

When u compile a program using command line, u add the references
using /r switch. When you compile a program using Visual Studio, it
adds those references to our assembly, which are added using “Add
Reference” dialog box. While “using” statement facilitates us to use
classes without using their fully qualified names.
For example: if u have added a reference to “System.Data.SqlClient”
using “Add Reference” dialog box then u can use SqlConnection class
like this:
System.Data.SqlClient.SqlConnection
But if u add a “using System.Data.SqlClient” statement at the start
of ur code then u can directly use SqlConnection class.
On the other hand if u add a reference using “using
System.Data.SqlClient” statement, but don’t add it using “Add
Reference” dialog box, Visual Studio will give error message while
we compile the program.