Redundancy in Using Directives

Question

I'm new to ASP.NET development and there is something I quite don't grasp regarding using directives...

using System;
using System.Data;
using System.Data.SqlClient;

In the above code example, I am curious as to why we must explicitly list System.Data and System.Data.SqlClient when they are already included in the first statement using System;. It seems redundant that we must specify namespaces that are included in parent namespaces. I know there is probably a very simple explanation to this question, but I've been unable to find it with my search.

Answer 1

using System;

doesn't include the namespace System.Data. Those are separate namespaces. Thus, you need both if you intend to use classes defined in both of those namespaces.