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10.6.3 Virtual, sealed, override, and abstract accessors

Visual Studio .NET 2003

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 (Section 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 Section 10.5.3, Section 10.5.4, Section 10.5.5, and Section 10.5.6 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.

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.

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