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:
  ?
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