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A class-declaration is a type-declaration (§9.5) that declares a new class.
attributesopt class-modifiersopt class identifier class-baseopt class-body ;opt
A class-declaration consists of an optional set of attributes (§17), followed by an optional set of class-modifiers (§10.1.1), followed by the keyword class and an identifier that names the class, followed by an optional class-base specification (§10.1.2), followed by a class-body (§10.1.3), optionally followed by a semicolon.
A class-declaration may optionally include a sequence of class modifiers:
It is an error for the same modifier to appear multiple times in a class declaration.
The new modifier is only permitted on nested classes. It specifies that the class hides an inherited member by the same name, as described in §10.2.2.
The public, protected, internal, and private modifiers control the accessibility of the class. Depending on the context in which the class declaration occurs, some of these modifiers may not be permitted (§3.5.1).
The abstract and sealed modifiers are discussed in the following sections.
The abstract modifier is used to indicate that a class is incomplete and that it is intended to be used only as a base class. An abstract class differs from a non-abstract class is the following ways:
An abstract class cannot be instantiated directly, and it is an error to use the new operator on an abstract class. While it is possible to have variables and values whose compile-time types are abstract, such variables and values will necessarily either be null or contain references to instances of non-abstract classes derived from the abstract types.
An abstract class is permitted (but not required) to contain abstract members.
An abstract class cannot be sealed.
When a non-abstract class is derived from an abstract class, the non-abstract class must include actual implementations of all inherited abstract members. Such implementations are provided by overriding the abstract members. In the example
class B: A
the abstract class A introduces an abstract method F. Class B introduces an additional method G, but since it doesn’t provide an implementation of F, B must also be declared abstract. Class C overrides F and provides an actual implementation. Since there are no abstract members in C, C is permitted (but not required) to be non-abstract.
The sealed modifier is used to prevent derivation from a class. An error occurs if a sealed class is specified as the base class of another class.
A sealed class cannot also be an abstract class.
The sealed modifier is primarily used to prevent unintended derivation, but it also enables certain run-time optimizations. In particular, because a sealed class is known to never have any derived classes, it is possible to transform virtual function member invocations on sealed class instances into non-virtual invocations.
A class declaration may include a class-base specification, which defines the direct base class of the class and the interfaces (13) implemented by the class.
: class-type , interface-type-list
interface-type-list , interface-type
When a class-type is included in the class-base, it specifies the direct base class of the class being declared. If a class declaration has no class-base, or if the class-base lists only interface types, the direct base class is assumed to be object. A class inherits members from its direct base class, as described in §10.2.1.
In the example
class B: A
class A is said to be the direct base class of B, and B is said to be derived from A. Since A does not explicitly specify a direct base class, its direct base class is implicitly object.
The direct base class of a class type must be at least as accessible as the class type itself (§3.5.4). For example, it is an error for a public class to derive from a private or internal class.
The direct base class of a class type must not be any of the following types: System.Array, System.Delegate, System.Enum, or System.ValueType.
The base classes of a class are the direct base class and its base classes. In other words, the set of base classes is the transitive closure of the direct base class relationship. Referring to the example above, the base classes of B are A and object.
Except for class object, every class has exactly one direct base class. The object class has no direct base class and is the ultimate base class of all other classes.
When a class B derives from a class A, it is an error for A to depend on B. A class directly depends on its direct base class (if any) and directly depends on the class within which it is immediately nested (if any). Given this definition, the complete set of classes upon which a class depends is the transitive closure of the directly depends on relationship.
class A: B
class B: C
class C: A
is in error because the classes circularly depend on themselves. Likewise, the example
class A: B.C
public class C
is in error because A depends on B.C (its direct base class), which depends on B (its immediately enclosing class), which circularly depends on A.
Note that a class does not depend on the classes that are nested within it. In the example
class B: A
B depends on A (because A is both its direct base class and its immediately enclosing class), but A does not depend on B (since B is neither a base class nor an enclosing class of A). Thus, the example is valid.
It is not possible to derive from a sealed class. In the example
sealed class A
class B: A // Error, cannot derive from a sealed class
class B is in error because it attempts to derive from the sealed class A.
A class-base specification may include a list of interface types, in which case the class is said to implement the given interface types. Interface implementations are discussed further in §13.4.
The class-body of a class defines the members of the class.
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