17.6.3 Virtual, sealed, override, and abstrac

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 (§17.1
.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 §17.5.3,
§17.5.4, §17.5.5, and §17.5.6 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.
[Example: 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; }
}
C# LANGUAGE SPECIFICATION
246
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. end example]