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This chapter is from the book

3.8 Namespace and Type Names

Several contexts in a C# program require a namespace-name or a type-name to be specified. Either form of name is written as one or more identifiers separated by "." tokens.

namespace-name:
     namespace-or-type-name

type-name:
     namespace-or-type-name

namespace-or-type-name:
     identifier
     namespace-or-type-name . identifier

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

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

  • If the namespace-or-type-name consists of a single identifier, then the following happens.
  • If the namespace-or-type-name appears within the body of a class or struct declaration, then starting with that class or struct declaration and continuing with each enclosing class or struct declaration (if any), if a member with the given name exists, is accessible, and denotes a type, then the namespace-or-type-name refers to that member. Note that nontype members (constants, fields, methods, properties, indexers, operators, instance constructors, destructors, and static constructors) are ignored when determining the meaning of a namespace-or-type-name.

    Otherwise, 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 namespace contains a namespace member with the given name, then the namespace-or-type-name refers to that member and, depending on the member, is classified as a namespace or a type.
    • Otherwise, if the namespace has a corresponding namespace declaration enclosing the location where the namespace-or-type-name occurs, then the following occurs.
      • If the namespace declaration contains a using-alias-directive that associates the given name 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 with the given name, then the namespace-or-type-name refers to that type.

        Otherwise, if the namespaces imported by the using-namespace-directives of the namespace declaration contain more than one type with the given name, 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, where N is a namespace-or-type-name consisting of all identifiers but the rightmost one, and I is the rightmost identifier. 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 is resolved as follows.
    • If N is a namespace and I is the name of an accessible member of that namespace, then N.I refers to that member and, depending on the member, is classified as a namespace or a type.

      If N is a class or struct type and I is the name of an accessible type in N, then N.I refers to that type.

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

3.8.1 Fully 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 following example 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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