ECMA-334: 16.3 Extern alias directives

C# Language Specification

16. Namespaces 16.1 Compilation units 16.2 Namespace declarations 16.3 Extern alias directives 16.4 Using directives 16.5 Namespace members 16.6 Type declarations 16.7 Qualified alias member

© 2006 ECMA International

16.3 Extern alias directives

An extern-alias-directive introduces an identifier that serves as an alias for an externally defined namespace. The specification of the aliased namespace is external to the source code of the program.

extern-alias-directives:

extern-alias-directive

extern-alias-directives extern-alias-directive

extern-alias-directive:

extern alias identifier ;

The scope of an extern-alias-directive extends over the using-directives, global-attributes and namespace-member-declarations of its immediately containing compilation-unit or namespace-body. An extern-alias-directive contributes its name to the alias declaration space of the containing compilation unit or namespace body (§10.3) and not to the declaration space of the containing namespace.

Within a compilation unit or namespace body that contains an extern-alias-directive, the identifier introduced by the extern-alias-directive can be used to reference the aliased namespace. It is a compile-time error for the identifier to be the word global.

Within C# source code, a type is declared a member of a single namespace. However, a namespace hierarchy referenced by an extern alias may contain types that are also members of other namespaces. For example, if A and B are extern aliases, the names A::X, B::C.Y and global::D.Z may, depending on the external specification supported by the particular compiler, all refer to the same type.

The alias introduced by an extern-alias-directive is very similar to the alias introduced by a using-alias-directive. See §16.4.1 for more detailed discussion of extern-alias-directives and using-alias-directives.

Like get and set in property accessors, alias is not a keyword (§9.4.3). The word alias only has special meaning when it immediately follows the extern keyword in an extern-alias-directive. [Example: In fact an extern alias could use the identifier alias as its name:

extern alias alias;

end example]