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5. Data Types
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prefix or postfix increment or decrement operators, or any of the compound assignment operators, both the get and set accessors must be declared.
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Modifiers
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The modifiers applicable to indexers depend on the context of their declaration. Table 5-18 summarizes the available modifiers for each context.
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Table 5-18. Indexer Declaration Modifier Availability
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Indexer Declaration Context Member of Class Accessibility public protected private internal protected internal Inheritance new abstract sealed virtual override Other N/A readonly N/A volatile static Static indexers are not permitted. N/A extern When the extern modifier is used, the get and set accessor bodies should be empty statements.
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Indexers as Members of Interfaces
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Member of Member Interface Struct (implicit) N/A N/A N/A N/A
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N/A N/A N/A N/A N/A N/A N/A N/A N/A
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N/A N/A N/A N/A N/A N/A N/A N/A
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Indexers can be declared in interfaces. As with properties, the get and set accessors must be specified as an empty statement. To implement a read-only or write-only indexer, simply omit the accessor that is not required. When implementing the interface, explicit interface implementation must be used to identify the indexer implementation. For example:
5. Data Types public interface IMyInterface { string this [int index] {get; set;} } public class MyClass : IMyInterface { string IMyInterface.this [int index] { get { // implementation } set { // implementation } } }
The problem with this approach is that the indexer cannot be accessed through an instance of the class, only through an instance of the interface. For example:
// This is a compile-time error MyClass a = new MyClass(); string b = a[1]; // The following statements are valid MyClass p = new MyClass(); string q = ((IMyInterface)p)[1]; // Cast to an interface instance IMyInterface x = new MyClass(); string y = x[1];
Operators
In this section, we'll consistently refer to this member type as an operator member to avoid confusion with regular operators such as + and !=. The operator member enables a programmer to specify the behavior when an instance of a class or struct is used in either of the following ways:
Is used in conjunction with an operator such as + or !=. This is called operator overloading. Is implicitly or explicitly converted to another type, known as custom conversion.
Operator members provide syntactic convenience resulting in clean and logical code when used correctly; however, when used inappropriately or in idiosyncratic ways, operator members result in code that is difficult to understand and types that are difficult to use. For example, it makes sense for a class CalendarDay that represents a calendar day to support the ++ operator to increment the day being represented, but use of the + operator to add two CalendarDay instances together is not logical and constitutes confusing behavior.
Declaration
The declaration of an operator member takes the following general form: [attributes] [modifiers] operator operator-token ( parameters ) { body }
5. Data Types
The type of operator member determines the specific declaration syntax required, but there are a number of restrictions common to all operator members:
Operator members are always associated with types and must be public and static. The only optional modifier is extern, in which case the body of the operator member must be an empty statement. Arguments to operator members cannot be ref or out parameters. Derived classes inherit operator members but cannot override or hide them. Operator members cannot return void.
There are three categories of operator members: unary, binary, and conversion. We discuss each of these in the following sections.
Unary operators
Unary operator members allow the following operators to be overloaded: +, -, !, ~, ++, --, true, and false. The true and false operators must be implemented as a pair, or a compiler error will occur. Unary operator members take a single argument of the type the operator member is declared in. The return type depends on the overloaded operator. Table 5-19 summarizes the return type for each operator.
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