10.5Member access
Declarations of members allow control over member access. The accessibility
of a member is established by
the declared accessibility (?0.5.1) of the member combined with the
accessibility of the immediately
containing type, if any.
When access to a particular member is allowed, the member is said to be
accessible. Conversely, when
access to a particular member is disallowed, the member is said to be
inaccessible. Access to a member is
permitted when the textual location in which the access takes place is
included in the accessibility domain
(?0.5.2) of the member.
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10.5.1 Declared accessibility
The declared accessibility of a member can be one of the following:
?Public, which is selected by including a public modifier in the member
declaration. The intuitive
meaning of public is .access not limited..
?Protected, which is selected by including a protected modifier in the
member declaration. The
intuitive meaning of protected is .access limited to the containing class
or types derived from the
containing class..
?Internal, which is selected by including an internal modifier in the
member declaration. The intuitive
meaning of internal is .access limited to this program..
?Protected internal, which is selected by including both a protected and
an internal modifier in the
member declaration. The intuitive meaning of protected internal is .access
limited to this program
or types derived from the containing class..
?Private, which is selected by including a private modifier in the member
declaration. The intuitive
meaning of private is .access limited to the containing type..
Depending on the context in which a member declaration takes place, only
certain types of declared
accessibility are permitted. Furthermore, when a member declaration does
not include any access modifiers,
the context in which the declaration takes place determines the default
declared accessibility.
?Namespaces implicitly have public declared accessibility. No access
modifiers are allowed on
namespace declarations.
?Types declared in compilation units or namespaces can have public or
internal declared
accessibility and default to internal declared accessibility.
?Class members can have any of the five kinds of declared accessibility
and default to private declared
accessibility. ([Note: A type declared as a member of a class can have any
of the five kinds of declared
accessibility, whereas a type declared as a member of a namespace can have
only public or internal
declared accessibility. end note])
?Struct members can have public, internal, or private declared
accessibility and default to
private declared accessibility because structs are implicitly sealed.
Struct members introduced in a
struct (that is, not inherited by that struct) cannot have protected or
protected internal declared
accessibility. ([Note: A type declared as a member of a struct can have
public, internal, or
private declared accessibility, whereas a type declared as a member of a
namespace can have only
public or internal declared accessibility. end note])
?Interface members implicitly have public declared accessibility. No
access modifiers are allowed on
interface member declarations.
?Enumeration members implicitly have public declared accessibility. No
access modifiers are allowed
on enumeration member declarations.
10.5.2 Accessibility domains
The accessibility domain of a member consists of the (possibly disjoint)
sections of program text in which
access to the member is permitted. For purposes of defining the
accessibility domain of a member, a member
is said to be top-level if it is not declared within a type, and a member
is said to be nested if it is declared
within another type. Furthermore, the text of an assembly is defined as all
source text contained in all source
files of that assembly, and the source text of a type is defined as all
source text contained between the
opening and closing .{. and .}. tokens in the class-body, struct-body,
interface-body, or enum-body of the
type (including, possibly, types that are nested within the type).
The accessibility domain of a predefined type (such as object, int, or
double) is unlimited.
The accessibility domain of a top-level type T that is declared in a
program P is defined as follows:
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?If the declared accessibility of T is public, the accessibility domain of
T is the program text of P and
any program that references P.
?If the declared accessibility of T is internal, the accessibility domain
of T is the program text of P.
[Note: From these definitions it follows that the accessibility domain of a
top-level type is always at least the
program text of the program in which that type is declared. end note]
The accessibility domain of a nested member M declared in a type T within a
program P, is defined as
follows (noting that M itself may possibly be a type):
?If the declared accessibility of M is public, the accessibility domain of
M is the accessibility domain
of T.
?If the declared accessibility of M is protected internal, let D be the
union of the program text of P
and the program text of any type derived from T, which is declared outside
P. The accessibility domain
of M is the intersection of the accessibility domain of T with D.
?If the declared accessibility of M is protected, let D be the union of
the program text of T and the
program text of any type derived from T. The accessibility domain of M is
the intersection of the
accessibility domain of T with D.
?If the declared accessibility of M is internal, the accessibility domain
of M is the intersection of the
accessibility domain of T with the program text of P.
?If the declared accessibility of M is private, the accessibility domain
of M is the program text of T.
[Note: From these definitions it follows that the accessibility domain of a
nested member is always at least
the program text of the type in which the member is declared. Furthermore,
it follows that the accessibility
domain of a member is never more inclusive than the accessibility domain of
the type in which the member
is declared. end note]
[Note: In intuitive terms, when a type or member M is accessed, the
following steps are evaluated to ensure
that the access is permitted:
?First, if M is declared within a type (as opposed to a compilation unit
or a namespace), a compile-time
error occurs if that type is not accessible.
?Then, if M is public, the access is permitted.
?Otherwise, if M is protected internal, the access is permitted if it
occurs within the program in
which M is declared, or if it occurs within a class derived from the class
in which M is declared and takes
place through the derived class type (?0.5.3).
?Otherwise, if M is protected, the access is permitted if it occurs within
the class in which M is declared,
or if it occurs within a class derived from the class in which M is
declared and takes place through the
derived class type (?0.5.3).
?Otherwise, if M is internal, the access is permitted if it occurs within
the program in which M is
declared.
?Otherwise, if M is private, the access is permitted if it occurs within
the type in which M is declared.
?Otherwise, the type or member is inaccessible, and a compile-time error
occurs.
end note]
[Example: In the example
public class A
{
public static int X;
internal static int Y;
private static int Z;
}
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internal class B
{
public static int X;
internal static int Y;
private static int Z;
public class C
{
public static int X;
internal static int Y;
private static int Z;
}
private class D
{
public static int X;
internal static int Y;
private static int Z;
}
}
the classes and members have the following accessibility domains:
?The accessibility domain of A and A.X is unlimited.
?The accessibility domain of A.Y, B, B.X, B.Y, B.C, B.C.X, and B.C.Y is
the program text of the
containing program.
?The accessibility domain of A.Z is the program text of A.
?The accessibility domain of B.Z and B.D is the program text of B,
including the program text of B.C
and B.D.
?The accessibility domain of B.C.Z is the program text of B.C.
?The accessibility domain of B.D.X and B.D.Y is the program text of B,
including the program text of
B.C and B.D.
?The accessibility domain of B.D.Z is the program text of B.D.
As the example illustrates, the accessibility domain of a member is never
larger than that of a containing
type. For example, even though all X members have public declared
accessibility, all but A.X have
accessibility domains that are constrained by a containing type. end
example]
As described in ?0.4, all members of a base class, except for instance
constructors, destructors, and static
constructors are inherited by derived types. This includes even private
members of a base class. However,
the accessibility domain of a private member includes only the program text
of the type in which the
member is declared. [Example: In the example
class A
{
int x;
static void F(B b) {
b.x = 1; // Ok
}
}
class B: A
{
static void F(B b) {
b.x = 1; // Error, x not accessible
}
}
the B class inherits the private member x from the A class. Because the
member is private, it is only
accessible within the class-body of A. Thus, the access to b.x succeeds in
the A.F method, but fails in the
B.F method. end example]
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10.5.3 Protected access for instance members
When a protected instance member is accessed outside the program text of
the class in which it is
declared, and when a protected internal instance member is accessed outside
the program text of the
program in which it is declared, the access is required to take place
through an instance of the derived class
type in which the access occurs. Let B be a base class that declares a
protected instance member M, and let D
be a class that derives from B. Within the class-body of D, access to M can
take one of the following forms:
?An unqualified type-name or primary-expression of the form M.
?A primary-expression of the form E.M, provided the type of E is D or a
class derived from D.
?A primary-expression of the form base.M.
In addition to these forms of access, a derived class can access a
protected instance constructor of a base
class in a constructor-initializer (?7.10.1).
[Example: In the example
public class A
{
protected int x;
static void F(A a, B b) {
a.x = 1; // Ok
b.x = 1; // Ok
}
}
public class B: A
{
static void F(A a, B b) {
a.x = 1; // Error, must access through instance of B
b.x = 1; // Ok
}
}
within A, it is possible to access x through instances of both A and B,
since in either case the access takes
place through an instance of A or a class derived from A. However, within
B, it is not possible to access x
through an instance of A, since A does not derive from B. end example]
10.5.4 Accessibility constraints
Several constructs in the C# language require a type to be at least as
accessible as a member or another type.
A type T is said to be at least as accessible as a member or type M if the
accessibility domain of T is a
superset of the accessibility domain of M. In other words, T is at least as
accessible as M if T is accessible in
all contexts in which M is accessible.
The following accessibility constraints exist:
?The direct base class of a class type must be at least as accessible as
the class type itself.
?The explicit base interfaces of an interface type must be at least as
accessible as the interface type itself.
?The return type and parameter types of a delegate type must be at least
as accessible as the delegate type
itself.
?The type of a constant must be at least as accessible as the constant
itself.
?The type of a field must be at least as accessible as the field itself.
?The return type and parameter types of a method must be at least as
accessible as the method itself.
?The type of a property must be at least as accessible as the property
itself.
?The type of an event must be at least as accessible as the event itself.
?The type and parameter types of an indexer must be at least as accessible
as the indexer itself.
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?The return type and parameter types of an operator must be at least as
accessible as the operator itself.
?The parameter types of an instance constructor must be at least as
accessible as the instance constructor
itself.
[Example: In the example
class A {.}
public class B: A {.}
the B class results in a compile-time error because A is not at least as
accessible as B. end example]
[Example: Likewise, in the example
class A {.}
public class B
{
A F() {.}
internal A G() {.}
public A H() {.}
}
the H method in B results in a compile-time error because the return type A
is not at least as accessible as the
method. end example]
