Given a type named:

System.Data.SqlClient.SqlConnection

The following works:

using System.Data.SqlClient;

...

         SqlConnection connection;

But this is an error:

using System.Data;

...

         SqlClient.SqlConnection connection;

Why?

Well, the rule is that C# only imports the types in the namespace mentioned in the "using" statement.

Back in the early days of C#, we had a slightly different rule, which I *think* also imported namespaces as well as types, but with that behavior, users were getting into situations where they had name collisions and were having difficulty figuring out what was going on (and, perhaps, coming up with workarounds - we obviously didn't have the global namespace operator coming in Whidbey), so we decided to limit the number of things that go into the global namespace.