Virtual, sealed, override, and abstract accessors. A virtual property declaration specifies that the accessors of the property are virtual

A virtual property declaration specifies that the accessors of the property are virtual. The virtual modifier applies to both accessors of a read-write property—it is not possible for only one accessor of a read-write property to be virtual.

An abstract property declaration specifies that the accessors of the property are virtual, but does not provide an actual implementation of the accessors. Instead, non-abstract derived classes are required to provide their own implementation for the accessors by overriding the property. Because an accessor for an abstract property declaration provides no actual implementation, its accessor-body simply consists of a semicolon.

A property declaration that includes both the abstract and override modifiers specifies that the property is abstract and overrides a base property. The accessors of such a property are also abstract.

Abstract property declarations are only permitted in abstract classes (§10.1.1.1).The accessors of an inherited virtual property can be overridden in a derived class by including a property declaration that specifies an override directive. This is known as an overriding property declaration. An overriding property declaration does not declare a new property. Instead, it simply specializes the implementations of the accessors of an existing virtual property.

An overriding property declaration must specify the exact same accessibility modifiers, type, and name as the inherited property. If the inherited property has only a single accessor (i.e., if the inherited property is read-only or write-only), the overriding property must include only that accessor. If the inherited property includes both accessors (i.e., if the inherited property is read-write), the overriding property can include either a single accessor or both accessors.

An overriding property declaration may include the sealed modifier. Use of this modifier prevents a derived class from further overriding the property. The accessors of a sealed property are also sealed.

Except for differences in declaration and invocation syntax, virtual, sealed, override, and abstract accessors behave exactly like virtual, sealed, override and abstract methods. Specifically, the rules described in §10.6.3, §10.6.4, §10.6.5, and §10.6.6 apply as if accessors were methods of a corresponding form:

· A get accessor corresponds to a parameterless method with a return value of the property type and the same modifiers as the containing property.

· A set accessor corresponds to a method with a single value parameter of the property type, a void return type, and the same modifiers as the containing property.

In the example

abstract class A
{
int y;

public virtual int X {
get { return 0; }
}

public virtual int Y {
get { return y; }
set { y = value; }
}

public abstract int Z { get; set; }
}

X is a virtual read-only property, Y is a virtual read-write property, and Z is an abstract read-write property. Because Z is abstract, the containing class A must also be declared abstract.

A class that derives from A is show below:

class B: A
{
int z;

public override int X {
get { return base.X + 1; }
}

public override int Y {
set { base.Y = value < 0? 0: value; }
}

public override int Z {
get { return z; }
set { z = value; }
}
}

Here, the declarations of X, Y, and Z are overriding property declarations. Each property declaration exactly matches the accessibility modifiers, type, and name of the corresponding inherited property. The get accessor of X and the set accessor of Y use the base keyword to access the inherited accessors. The declaration of Z overrides both abstract accessors—thus, there are no outstanding abstract function members in B, and B is permitted to be a non-abstract class.

When a property is declared as an override, any overridden accessors must be accessible to the overriding code. In addition, the declared accessibility of both the property or indexer itself, and of the accessors, must match that of the overridden member and accessors. For example:

public class B
{
public virtual int P {
protected set {...}
get {...}
}
}

public class D: B
{
public override int P {
protected set {...} // Must specify protected here
get {...} // Must not have a modifier here
}
}

Events

An event is a member that enables an object or class to provide notifications. Clients can attach executable code for events by supplying event handlers.

Events are declared using event-declarations:

event-declaration:
attributes_opt event-modifiers_opt event type variable-declarators;
attributes_opt event-modifiers_opt event type member-name { event-accessor-declarations }

event-modifiers:
event-modifier
event-modifiers event-modifier

event-modifier:
new
public
protected
internal
private
static
virtual
sealed
override
abstract
extern

event-accessor-declarations:
add-accessor-declaration remove-accessor-declaration
remove-accessor-declaration add-accessor-declaration

add-accessor-declaration:
attributes_opt add block

remove-accessor-declaration:
attributes_opt remove block

An event-declaration may include a set of attributes (§17) and a valid combination of the four access modifiers (§10.3.5), the new (§10.3.4), static (§10.6.2), virtual (§10.6.3), override (§10.6.4), sealed (§10.6.5), abstract (§10.6.6), and extern (§10.6.7) modifiers.

Event declarations are subject to the same rules as method declarations (§10.6) with regard to valid combinations of modifiers.

The type of an event declaration must be a delegate-type (§4.2), and that delegate-type must be at least as accessible as the event itself (§3.5.4).

An event declaration may include event-accessor-declarations. However, if it does not, for non-extern, non-abstract events, the compiler supplies them automatically (§10.8.1); for extern events, the accessors are provided externally.

An event declaration that omits event-accessor-declarations defines one or more events—one for each of the variable-declarators. The attributes and modifiers apply to all of the members declared by such an event-declaration.

It is a compile-time error for an event-declaration to include both the abstract modifier and brace-delimited event-accessor-declarations.

When an event declaration includes an extern modifier, the event is said to be an external event. Because an external event declaration provides no actual implementation, it is an error for it to include both the extern modifier and event-accessor-declarations.

It is a compile-time error for a variable-declarator of an event declaration with an abstract or external modifier to include a variable-initializer.

An event can be used as the left-hand operand of the += and -= operators (§7.17.3). These operators are used, respectively, to attach event handlers to or to remove event handlers from an event, and the access modifiers of the event control the contexts in which such operations are permitted.

Since += and -= are the only operations that are permitted on an event outside the type that declares the event, external code can add and remove handlers for an event, but cannot in any other way obtain or modify the underlying list of event handlers.

In an operation of the form x += y or x -= y, when x is an event and the reference takes place outside the type that contains the declaration of x, the result of the operation has type void (as opposed to having the type of x, with the value of x after the assignment). This rule prohibits external code from indirectly examining the underlying delegate of an event.

The following example shows how event handlers are attached to instances of the Button class:

public delegate void EventHandler(object sender, EventArgs e);

public class Button: Control
{
public event EventHandler Click;
}

public class LoginDialog: Form
{
Button OkButton;
Button CancelButton;

public LoginDialog() {
OkButton = new Button(...);
OkButton.Click += new EventHandler(OkButtonClick);
CancelButton = new Button(...);
CancelButton.Click += new EventHandler(CancelButtonClick);
}

void OkButtonClick(object sender, EventArgs e) {
// Handle OkButton.Click event
}

void CancelButtonClick(object sender, EventArgs e) {
// Handle CancelButton.Click event
}
}

Here, the LoginDialog instance constructor creates two Button instances and attaches event handlers to the Click events.