Jon Jagger
jon@jaggersoft.com
Table of Contents 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Notes DownloadECMA-334 C# Language Specificationpreviousnextprevious at this levelnext at this level 17 Classesprevious at this levelnext at this level 17.6 Propertiesprevious at this levelnext at this level 17.6.3 Virtual, sealed, override, and abstract accessors Paragraph 11 A virtual property declaration specifies that the accessors of the property are virtual. 2 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. Paragraph 21 An abstract property declaration specifies that the accessors of the property are virtual, but does not provide an actual implementation of the accessors. 2 Instead, non-abstract derived classes are required to provide their own implementation for the accessors by overriding the property. 3 Because an accessor for an abstract property declaration provides no actual implementation, its accessor-body simply consists of a semicolon. Paragraph 31 A property declaration that includes both the abstract and override modifiers specifies that the property is abstract and overrides a base property. 2 The accessors of such a property are also abstract. Paragraph 41 Abstract property declarations are only permitted in abstract classes (§17.1.1.1). 2 The accessors of an inherited virtual property can be overridden in a derived class by including a property declaration that specifies an override directive. 3 This is known as an overriding property declaration. 4 An overriding property declaration does not declare a new property. 5 Instead, it simply specializes the implementations of the accessors of an existing virtual property. Paragraph 51 An overriding property declaration must specify the exact same accessibility modifiers, type, and name as the inherited property. 2 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. 3 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. Paragraph 61 An overriding property declaration may include the sealed modifier. 2 Use of this modifier prevents a derived class from further overriding the property. 3 The accessors of a sealed property are also sealed. Paragraph 71 Except for differences in declaration and invocation syntax, virtual, sealed, override, and abstract accessors behave exactly like virtual, sealed, override and abstract methods. 2 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: [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; }  
}  
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]
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