10.7 Scopes

王朝other·作者佚名  2006-01-10
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10.7 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. [Note: This does not, however,

remove the restriction imposed by

?0.3 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. end note] 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 (?6.4) with no

enclosing namespace-declaration is the entire program text.

?The scope of a namespace member declared by a

namespace-member-declaration within a namespacedeclaration

whose fully qualified name is N, is the namespace-body of every

namespace-declaration

whose fully qualified name is N or starts with N, followed by a period.

?The scope of a name defined or imported by a using-directive (?6.3)

extends over the namespacemember-

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 (?7.2) 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 (?0.5.2) of the

member.

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80

?The scope of a member declared by a struct-member-declaration (?8.2) is

the struct-body in which the

declaration occurs.

?The scope of a member declared by an enum-member-declaration (?1.3) is

the enum-body in which the

declaration occurs.

?The scope of a parameter declared in a method-declaration (?7.5) is the

method-body of that methoddeclaration.

?The scope of a parameter declared in an indexer-declaration (?7.8) is

the accessor-declarations of that

indexer-declaration.

?The scope of a parameter declared in an operator-declaration (?7.9) is

the block of that operatordeclaration.

?The scope of a parameter declared in a constructor-declaration (?7.10)

is the constructor-initializer

and block of that constructor-declaration.

?The scope of a label declared in a labeled-statement (?5.4) is the block

in which the declaration occurs.

?The scope of a local variable declared in a local-variable-declaration (?5

.5.1) is the block in which the

declaration occurs.

?The scope of a local variable declared in a switch-block of a switch

statement (?5.7.2) is the switchblock.

?The scope of a local variable declared in a for-initializer of a for

statement (?5.8.3) is the forinitializer,

the for-condition, the for-iterator, and the contained statement of the for

statement.

?The scope of a local constant declared in a local-constant-declaration (?5

.5.2) is the block in which the

declaration occurs. It is a compile-time error to refer to a local constant

in a textual position that

precedes its constant-declarator.

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.

[Example: For example

class A

{

void F() {

i = 1;

}

int i = 0;

}

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

Within the scope of a local variable, it is a compile-time error to refer

to the local variable in a textual

position that precedes the local-variable-declarator of the local variable.

[Example: For example

class A

{

int i = 0;

void F() {

i = 1; // Error, use precedes declaration

int i;

i = 2;

}

void G() {

int j = (j = 1); // Valid

}

void H() {

int a = 1, b = ++a; // Valid

}

}

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81

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 results in a

compile-time 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 valid because the use does not precede the local-variable-declarator.

In the H method, a subsequent

local-variable-declarator correctly refers to a local variable declared in

an earlier local-variable-declarator

within the same local-variable-declaration. end example]

[Note: 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 were 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. (In certain situations, but

not in the example above, this could lead to a compile-time error 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

using System;

class A {}

class Test

{

static void Main() {

string A = "hello, world";

string s = A; // expression context

Type t = typeof(A); // type context

Console.WriteLine(s); // writes "hello, world"

Console.WriteLine(t.ToString()); // writes "Type: A"

}

}

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. end note]

 
 
 
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