C# Language Specification

Name binding

Although each part of an extensible type must be declared within the same namespace, the parts are typically written within different namespace declarations. Thus, different using directives (§9.4) may be present for each part. When interpreting simple names (§7.5.2) within one part, only the using directives of the namespace declaration(s) enclosing that part are considered. This may result in the same identifier having different meanings in different parts:

namespace N

{
using List = System.Collections.ArrayList;

partial class A

{
List x; // x has type System.Collections.ArrayList
}
}

namespace N

{
using List = Widgets.LinkedList;

partial class A

{
List y; // y has type Widgets.LinkedList
}
}

3.