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3.8Namespace and type names


Several contexts in a C# program require a namespace-name or a type-name to be specified.

namespace-name:
namespace-or-type-name


type-name:
namespace-or-type-name


namespace-or-type-name:
identifier type-argument-listopt
namespace-or-type-name . identifier type-argument-listop
qualified-alias-member

A namespace-name is a namespace-or-type-name that refers to a namespace. Following resolution as described below, the namespace-or-type-name of a namespace-name must refer to a namespace, or otherwise a compile-time error occurs. No type arguments (§4.4.1) can be present in a namespace-name (only types can have type arguments).

A type-name is a namespace-or-type-name that refers to a type. Following resolution as described below, the namespace-or-type-name of a type-name must refer to a type, or otherwise a compile-time error occurs.

If the namespace-or-type-name is a qualified-alias-member its meaning is as described in §9.7. Otherwise, a namespace-or-type-name has one of four forms:



  • I

  • I1, ..., AK>

  • N.I

  • N.I1, ..., AK>

where I is a single identifier, N is a namespace-or-type-name and 1, ..., AK> is an optional type-argument-list. When no type-argument-list is specified, consider K to be zero.

The meaning of a namespace-or-type-name is determined as follows:



  • If the namespace-or-type-name is of the form I or of the form I1, ..., AK>:

  • If K is zero and the namespace-or-type-name appears within the body of a generic method declaration (§10.6) and if that declaration includes a type parameter (§10.1.3) with name I, then the namespace-or-type-name refers to that type parameter.

  • Otherwise, if the namespace-or-type-name appears within the body of a type declaration, then for each instance type T (§10.3.1), starting with the instance type of that type declaration and continuing with the instance type of each enclosing class or struct declaration (if any):

  • If K is zero and the declaration of T includes a type parameter with name I, then the namespace-or-type-name refers to that type parameter.

  • Otherwise, if T or any of its base types contain a nested accessible type having name I and K type parameters, then the namespace-or-type-name refers to that type constructed with the given type arguments. If there is more than one such type, the type declared within the more derived type is selected. Note that non-type members (constants, fields, methods, properties, indexers, operators, instance constructors, destructors, and static constructors) and type members with a different number of type parameters are ignored when determining the meaning of the namespace-or-type-name.

  • If the previous steps where unsuccessful then, for each namespace N, starting with the namespace in which the namespace-or-type-name occurs, continuing with each enclosing namespace (if any), and ending with the global namespace, the following steps are evaluated until an entity is located:

  • If the location where the namespace-or-type-name occurs is enclosed by a namespace declaration for N and the namespace declaration contains an extern-alias-directive or using-alias-directive that associates the name I with a namespace or type, then the namespace-or-type-name is ambiguous and a compile-time error occurs.

  • Otherwise, the namespace-or-type-name refers to the namespace named I in N.

  • Otherwise, if N contains an accessible type having name I and K type parameters, then:

  • If K is zero and the location where the namespace-or-type-name occurs is enclosed by a namespace declaration for N and the namespace declaration contains an extern-alias-directive or using-alias-directive that associates the name I with a namespace or type, then the namespace-or-type-name is ambiguous and a compile-time error occurs.

  • Otherwise, the namespace-or-type-name refers to the type constructed with the given type arguments.

  • Otherwise, if the location where the namespace-or-type-name occurs is enclosed by a namespace declaration for N:

  • If K is zero and the namespace declaration contains an extern-alias-directive or using-alias-directive that associates the name I with an imported namespace or type, then the namespace-or-type-name refers to that namespace or type.

  • Otherwise, if the namespaces imported by the using-namespace-directives of the namespace declaration contain exactly one type having name I and K type parameters, then the namespace-or-type-name refers to that type constructed with the given type arguments.

  • Otherwise, if the namespaces imported by the using-namespace-directives of the namespace declaration contain more than one type having name I and K type parameters, then the namespace-or-type-name is ambiguous and an error occurs.

  • Otherwise, the namespace-or-type-name is undefined and a compile-time error occurs.

  • Otherwise, the namespace-or-type-name is of the form N.I or of the form N.I1, ..., AK>. N is first resolved as a namespace-or-type-name. If the resolution of N is not successful, a compile-time error occurs. Otherwise, N.I or N.I1, ..., AK> is resolved as follows:

  • If K is zero and N refers to a namespace and N contains a nested namespace with name I, then the namespace-or-type-name refers to that nested namespace.

  • Otherwise, if N refers to a namespace and N contains an accessible type having name I and K type parameters, then the namespace-or-type-name refers to that type constructed with the given type arguments.

  • Otherwise, if N refers to a (possibly constructed) class or struct type and N contains a nested accessible type having name I and K type parameters, then the namespace-or-type-name refers to that type constructed with the given type arguments. If there is more than one such type, the type declared within the more derived type is selected.

  • Otherwise, N.I is an invalid namespace-or-type-name, and a compile-time error occurs.

A namespace-or-type-name is permitted to reference a static class (§10.1.1.3) only if

  • The namespace-or-type-name is the T in a namespace-or-type-name of the form T.I, or

  • The namespace-or-type-name is the T in a typeof-expression (§7.5.11) of the form typeof(T).

3.8.1Fully qualified names


Every namespace and type has a fully qualified name, which uniquely identifies the namespace or type amongst all others. The fully qualified name of a namespace or type N is determined as follows:

  • If N is a member of the global namespace, its fully qualified name is N.

  • Otherwise, its fully qualified name is S.N, where S is the fully qualified name of the namespace or type in which N is declared.

In other words, the fully qualified name of N is the complete hierarchical path of identifiers that lead to N, starting from the global namespace. Because every member of a namespace or type must have a unique name, it follows that the fully qualified name of a namespace or type is always unique.

The example below shows several namespace and type declarations along with their associated fully qualified names.

class A {} // A

namespace X // X


{
class B // X.B
{
class C {} // X.B.C
}

namespace Y // X.Y


{
class D {} // X.Y.D
}
}

namespace X.Y // X.Y


{
class E {} // X.Y.E
}


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