The System.Enum type

The type System.Enum is the abstract base class of all enum types (this is distinct and different from the underlying type of the enum type), and the members inherited from System.Enum are available in any enum type. A boxing conversion (§4.3.1) exists from any enum type to System.Enum, and an unboxing conversion (§4.3.2) exists from System.Enum to any enum type.

Note that System.Enum is not itself an enum-type. Rather, it is a class-type from which all enum-types are derived. The type System.Enum inherits from the type System.ValueType (§4.1.1), which, in turn, inherits from type object. At run-time, a value of type System.Enum can be null or a reference to a boxed value of any enum type.

Enum values and operations

Each enum type defines a distinct type; an explicit enumeration conversion (§6.2.2) is required to convert between an enum type and an integral type, or between two enum types. The set of values that an enum type can take on is not limited by its enum members. In particular, any value of the underlying type of an enum can be cast to the enum type, and is a distinct valid value of that enum type.

Enum members have the type of their containing enum type (except within other enum member initializers: see §14.3). The value of an enum member declared in enum type E with associated value v is (E)v.

The following operators can be used on values of enum types: ==,!=, <, >, <=, >= (§7.10.5), binary + (§7.8.4), binary ‑ (§7.8.5), ^, &, | (§7.11.2), ~ (§7.7.4), ++ and -- (§7.6.9 and §7.7.5).

Every enumtype automatically derives from the class System.Enum (which, in turn, derives from System.ValueType and object). Thus, inherited methods and properties of this class can be used on values of an enum type.

Delegates

Delegates enable scenarios that other languages—such as C++, Pascal, and Modula—have addressed with function pointers. Unlike C++ function pointers, however, delegates are fully object oriented, and unlike C++ pointers to member functions, delegates encapsulate both an object instance and a method.

A delegate declaration defines a class that is derived from the class System.Delegate. A delegate instance encapsulates an invocation list, which is a list of one or more methods, each of which is referred to as a callable entity. For instance methods, a callable entity consists of an instance and a method on that instance. For static methods, a callable entity consists of just a method. Invoking a delegate instance with an appropriate set of arguments causes each of the delegate’s callable entities to be invoked with the given set of arguments.

An interesting and useful property of a delegate instance is that it does not know or care about the classes of the methods it encapsulates; all that matters is that those methods be compatible (§15.1) with the delegate’s type. This makes delegates perfectly suited for “anonymous” invocation.

Дата добавления: 2015-11-16