Scopes - Name hiding

Scopes

The scope of a name is the region of program text within which it is possible to refer to the entity declared by the name without qualification of the name. Scopes can be nested, and an inner scope may redeclare the meaning of a name from an outer scope. (This does not, however, remove the restriction imposed by ‎3.3 that within a nested block it is not possible to declare a local variable with the same name as a local variable in an enclosing block.) The name from the outer scope is then said to be hidden in the region of program text covered by the inner scope, and access to the outer name is only possible by qualifying the name.

The scope of a namespace member declared by a namespace-member-declaration with no enclosing namespace-declaration is the entire program text of each compilation unit.

The scope of a namespace member declared by a namespace-member-declaration within a namespace-declaration whose fully qualified name is N is the namespace-body of every namespace-declaration whose fully qualified name is N or starts with the same sequence of identifiers as N.

The scope of a name defined or imported by a using-directive extends over the namespace-member-declarations of the compilation-unit or namespace-body in which the using-directive occurs. A using-directive may make zero or more namespace or type names available within a particular compilation-unit or namespace-body, but does not contribute any new members to the underlying declaration space. In other words, a using-directive is not transitive but rather affects only the compilation-unit or namespace-body in which it occurs.

The scope of a member declared by a class-member-declaration is the class-body in which the declaration occurs. In addition, the scope of a class member extends to the class-body of those derived classes that are included in the accessibility domain (‎3.5.2) of the member.

The scope of a member declared by a struct-member-declaration is the struct-body in which the declaration occurs.

The scope of a member declared by an enum-member-declaration is the enum-body in which the declaration occurs.

The scope of a parameter declared in a constructor-declaration is the constructor-initializer and block of that constructor-declaration.

The scope of a parameter declared in a method-declaration is the method-body of that method-declaration.

The scope of a parameter declared in an indexer-declaration is the accessor-declarations of that indexer-declaration.

The scope of a parameter declared in an operator-declaration is the block of that operator-declaration.

The scope of a local variable declared in a local-variable-declaration is the block in which the declaration occurs. It is an error to refer to a local variable in a textual position that precedes the variable-declarator of the local variable.

The scope of a local variable declared in a for-initializer of a for statement is the for-initializer, the for-condition, the for-iterator, and the contained statement of the for statement.

The scope of a label declared in a labeled-statement is the block in which the declaration occurs.

Within the scope of a namespace, class, struct, or enumeration member it is possible to refer to the member in a textual position that precedes the declaration of the member. For example

class A

int i = 0;
}

Here, it is valid for F to refer to i before it is declared.

Within the scope of a local variable, it is an error to refer to the local variable in a textual position that precedes the variable-declarator of the local variable. For example

class A

void G()

void H()
}

In the F method above, the first assignment to i specifically does not refer to the field declared in the outer scope. Rather, it refers to the local variable and it is in error because it textually precedes the declaration of the variable. In the G method, the use of j in the initializer for the declaration of j is legal because the use does not precede the variable-declarator. In the H method, a subsequent variable-declarator legally refers to a local variable declared in an earlier variable-declarator within the same local-variable-declaration.

The scoping rules for local variables are designed to guarantee that the meaning of a name used in an expression context is always the same within a block. If the scope of a local variable was to extend only from its declaration to the end of the block, then in the example above, the first assignment would assign to the instance variable and the second assignment would assign to the local variable, possibly leading to errors if the statements of the block were later to be rearranged.

The meaning of a name within a block may differ based on the context in which the name is used. In the example

class A

class Test

}

the name A is used in an expression context to refer to the local variable A and in a type context to refer to the class A.

Name hiding

The scope of an entity typically encompasses more program text than the declaration space of the entity. In particular, the scope of an entity may include declarations that introduce new declaration spaces containing entities of the same name. Such declarations cause the original entity to become hidden. Conversely, an entity is said to be visible when it is not hidden.

Name hiding occurs when scopes overlap through nesting and when scopes overlap through inheritance. The characteristics of the two types of hiding are described in the following sections.

Hiding through nesting

Name hiding through nesting can occur as a result of nesting namespaces or types within namespaces, as a result of nesting types within classes or structs, and as a result of parameter and local variable declarations.

In the example

class A

void G()
}

within the F method, the instance variable i is hidden by the local variable i, but within the G method, i still refers to the instance variable.

When a name in an inner scope hides a name in an outer scope, it hides all overloaded occurrences of that name. In the example

class Outer
{
static void F(int i)

static void F(string s)

class Inner

static void F(long l)
}
}

the call F(1) invokes the F declared in Inner because all outer occurrences of F are hidden by the inner declaration. For the same reason, the call F('Hello') is in error.

Hiding through inheritance

Name hiding through inheritance occurs when classes or structs redeclare names that were inherited from base classes. This type of name hiding takes one of the following forms:

A constant, field, property, event, or type introduced in a class or struct hides all base class members with the same name.

A method introduced in a class or struct hides all non-method base class members with the same name, and all base class methods with the same signature (method name and parameter count, modifiers, and types).

An indexer introduced in a class or struct hides all base class indexers with the same signature (parameter count and types).

The rules governing operator declarations (‎10.9) make it impossible for a derived class to declare an operator with the same signature as an operator in a base class. Thus, operators never hide one another.

Contrary to hiding a name from an outer scope, hiding an accessible name from an inherited scope causes a warning to be reported. In the example

class Base
{
public void F()
}

class Derived: Base
{
public void F() // Warning, hiding an inherited name
}

the declaration of F in Derived causes a warning to be reported. Hiding an inherited name is specifically not an error, since that would preclude separate evolution of base classes. For example, the above situation might have come about because a later version of Base introduced an F method that wasn't present in an earlier version of the class. Had the above situation been an error, then any change made to a base class in a separately versioned class library could potentially cause derived classes to become invalid.

The warning caused by hiding an inherited name can be eliminated through use of the new modifier:

class Base
{
public void F()
}

class Derived: Base
{
new public void F()
}

The new modifier indicates that the F in Derived is "new", and that it is indeed intended to hide the inherited member.

A declaration of a new member hides an inherited member only within the scope of the new member.

class Base
{
public static void F()
}

class Derived: Base
{
new private static void F() // Hides Base.F in Derived only
}

class MoreDerived: Derived
// Invokes Base.F
}

In the example above, the declaration of F in Derived hides the F that was inherited from Base, but since the new F in Derived has private access, its scope does not extend to MoreDerived. Thus, the call F() in MoreDerived.G is valid and will invoke Base.F.