Indexer access

For an indexer access, the primary-no-array-creation-expression of the element-access must be a variable or value of a class, struct, or interface type, and this type must implement one or more indexers that are applicable with respect to the argument-list of the element-access.

The binding-time processing of an indexer access of the form P[A], where P is a primary-no-array-creation-expression of a class, struct, or interface type T, and A is an argument-list, consists of the following steps:

· The set of indexers provided by T is constructed. The set consists of all indexers declared in T or a base type of T that are not override declarations and are accessible in the current context (§3.5).

· The set is reduced to those indexers that are applicable and not hidden by other indexers. The following rules are applied to each indexer S.I in the set, where S is the type in which the indexer I is declared:

o If I is not applicable with respect to A (§7.5.3.1), then I is removed from the set.

o If I is applicable with respect to A (§7.5.3.1), then all indexers declared in a base type of S are removed from the set.

o If I is applicable with respect to A (§7.5.3.1) and S is a class type other than object, all indexers declared in an interface are removed from the set.

· If the resulting set of candidate indexers is empty, then no applicable indexers exist, and a binding-time error occurs.

· The best indexer of the set of candidate indexers is identified using the overload resolution rules of §7.5.3. If a single best indexer cannot be identified, the indexer access is ambiguous, and a binding-time error occurs.

· The index expressions of the argument-list are evaluated in order, from left to right. The result of processing the indexer access is an expression classified as an indexer access. The indexer access expression references the indexer determined in the step above, and has an associated instance expression of P and an associated argument list of A.

Depending on the context in which it is used, an indexer access causes invocation of either the get-accessor or the set-accessor of the indexer. If the indexer access is the target of an assignment, the set-accessor is invoked to assign a new value (§7.17.1). In all other cases, the get-accessor is invoked to obtain the current value (§7.1.1).

This access

A this-access consists of the reserved word this.

this-access:
this

A this-access is permitted only in the block of an instance constructor, an instance method, or an instance accessor. It has one of the following meanings:

· When this is used in a primary-expression within an instance constructor of a class, it is classified as a value. The type of the value is the instance type (§10.3.1) of the class within which the usage occurs, and the value is a reference to the object being constructed.

· When this is used in a primary-expression within an instance method or instance accessor of a class, it is classified as a value. The type of the value is the instance type (§10.3.1) of the class within which the usage occurs, and the value is a reference to the object for which the method or accessor was invoked.

· When this is used in a primary-expression within an instance constructor of a struct, it is classified as a variable. The type of the variable is the instance type (§10.3.1) of the struct within which the usage occurs, and the variable represents the struct being constructed. The this variable of an instance constructor of a struct behaves exactly the same as an out parameter of the struct type—in particular, this means that the variable must be definitely assigned in every execution path of the instance constructor.

· When this is used in a primary-expression within an instance method or instance accessor of a struct, it is classified as a variable. The type of the variable is the instance type (§10.3.1) of the struct within which the usage occurs.

o If the method or accessor is not an iterator (§10.14), the this variable represents the struct for which the method or accessor was invoked, and behaves exactly the same as a ref parameter of the struct type.

o If the method or accessor is an iterator, the this variable represents a copy of the struct for which the method or accessor was invoked, and behaves exactly the same as a value parameter of the struct type.

Use of this in a primary-expression in a context other than the ones listed above is a compile-time error. In particular, it is not possible to refer to this in a static method, a static property accessor, or in a variable-initializer of a field declaration.

Base access

A base-access consists of the reserved word base followed by either a “.” token and an identifier or an argument-list enclosed in square brackets:

base-access:
base. identifier
base [ argument-list ]

A base-access is used to access base class members that are hidden by similarly named members in the current class or struct. A base-access is permitted only in the block of an instance constructor, an instance method, or an instance accessor. When base.I occurs in a class or struct, I must denote a member of the base class of that class or struct. Likewise, when base[E] occurs in a class, an applicable indexer must exist in the base class.

At binding-time, base-access expressions of the form base.I and base[E] are evaluated exactly as if they were written ((B)this).I and ((B)this)[E], where B is the base class of the class or struct in which the construct occurs. Thus, base.I and base[E] correspond to this.I and this[E], except this is viewed as an instance of the base class.

When a base-access references a virtual function member (a method, property, or indexer), the determination of which function member to invoke at run-time (§7.5.4) is changed. The function member that is invoked is determined by finding the most derived implementation (§10.6.3) of the function member with respect to B (instead of with respect to the run-time type of this, as would be usual in a non-base access). Thus, within an override of a virtual function member, a base-access can be used to invoke the inherited implementation of the function member. If the function member referenced by a base-access is abstract, a binding-time error occurs.

Дата добавления: 2015-11-16