10.7.1 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.
10.7.1.1 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.
[Example: In the example
class A
{
int i = 0;
void F() {
int i = 1;
}
void G() {
i = 1;
}
}
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. end example]
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When a name in an inner scope hides a name in an outer scope, it hides all
overloaded occurrences of that
name. [Example: In the example
class Outer
{
static void F(int i) {}
static void F(string s) {}
class Inner
{
void G() {
F(1); // Invokes Outer.Inner.F
F("Hello"); // Error
}
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") results in a
compile-time error. end example]
10.7.1.2 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 (?7.9) 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. [Example: 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. end example]
The warning caused by hiding an inherited name can be eliminated through
use of the new modifier:
[Example:
class Base
{
public void F() {}
}
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83
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. end example]
A declaration of a new member hides an inherited member only within the
scope of the new member.
[Example:
class Base
{
public static void F() {}
}
class Derived: Base
{
new private static void F() {} // Hides Base.F in Derived only
}
class MoreDerived: Derived
{
static void G() { F(); } // 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. end example]