Namespaces and subnamespaces in C#

Question

begginers question about C#.

In every program I have to include several namespaces, like:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

Why do I have to include:

using System.Collections.Generic;
using System.Linq;
using System.Text;

... since they are included with first one:

using System;

Thank you in advance!

Answer 1

Because nested namespaces are not included with parent one. See using directive documentation for details

A using directive does not give you access to any namespaces that are nested in the namespace you specify.

Answer 2

System and System.IO namespaces are different.

You can treat "subnamespace" as parent-child relationship in the object model. If you have access to the "Car" object does not mean that you have access to car's wheels.

System is a huge namespace that contains hundreds of nested namespace and thousands of classes. You should specify all nested namespaces separately to state what part of the module are you interested in.