This chapter is from the book
This chapter is from the book
This chapter is from the book
3.7 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 (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 (§9.4) with no enclosing namespace-declaration is the entire program text.
- The scope of a namespace member declared by a namespace-member-declaration within a 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 N, followed by a period.
- The scope of a name defined or imported by a using-directive (§9.3) 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 it 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 (§10.2) 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 (§3.5.2) of the member.
- The scope of a member declared by a struct-member-declaration (§11.2) is the struct-body in which the declaration occurs.
- The scope of a member declared by an enum-member-declaration (§14.3) is the enum-body in which the declaration occurs.
- The scope of a parameter declared in a method-declaration (§10.5) is the method-body of that method-declaration.
- The scope of a parameter declared in an indexer-declaration (§10.8) is the accessor-declarations of that indexer-declaration.
- The scope of a parameter declared in an operator-declaration (§10.9) is the block of that operator-declaration.
- The scope of a parameter declared in a constructor-declaration (§10.10) is the constructor-initializer and block of that constructor-declaration.
- The scope of a label declared in a labeled-statement (§8.4) is the block in which the declaration occurs.
- The scope of a local variable declared in a local-variable-declaration (§8.5.1) is the block in which the declaration occurs.
- The scope of a local variable declared in a switch-block of a switch statement (§8.7.2) is the switch-block.
- The scope of a local variable declared in a for-initializer of a for statement (§8.8.3) is the for-initializer, 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 (§8.5.2) 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. In the following example, it is valid for F to refer to i before it is declared.
class A
{ void F() { i = 1; } int i = 0; }
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. For example, in the F method, the first assignment to i specifically does not refer to the field declared in the outer scope.
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
}
}
Rather, it refers to the local variable and results in a compile-time error because it textually precedes the declaration of the variable. In the G method, using j in the initializer for the declaration of j is valid because its 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.
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 previous example the first assignment would assign to the instance variable and the second assignment would assign to the local variable, possibly leading to compile-time errors if the statements of the block were later rearranged.
The meaning of a name within a block may differ based on the context in which the name is used. In the following example, 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.
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); // writes "A"
}
}
3.7.1 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.
3.7.1.1 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.
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.
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
{
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.
3.7.1.2 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 nonmethod 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 following example, the declaration of F in Derived causes a warning to be reported.
class Base
{ public void F() {}
} class Derived: Base
{ public void F() {} // Warning, hiding an inherited name }
Hiding an inherited name is specifically not an error because that would preclude separate evolution of base classes. For example, the previous situation might have come about because a later version of Base introduced an F method that was not present in an earlier version of the class. Had the previous 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 by using 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. In the following 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 }
the declaration of F in Derived hides the F that was inherited from Base, but because 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.