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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Hyperlinked ECMA C# Language Specification</title><meta name="author" content="Jon Jagger" /><link rel="stylesheet" href="ecma334.css"></link></head><body><div align="right"><em><a href="http://www.jaggersoft.com">Jon Jagger</a></em></div><div align="right"><a href="mailto:jon@jaggersoft.com">jon@jaggersoft.com</a></div><form method="get" action="http://search.atomz.com/search/"><input size="30" name="sp-q"></input><input type="submit" value="Search C# Spec"></input><input type="hidden" name="sp-a" value="sp10024177"></input><input type="hidden" name="sp-f" value="ISO-8859-1"></input></form><a href="toc.htm">Table of Contents</a> <a href="1.htm">1</a> <a href="2.htm">2</a> <a href="3.htm">3</a> <a href="4.htm">4</a> <a href="5.htm">5</a> <a href="6.htm">6</a> <a href="7.htm">7</a> <a href="8.htm">8</a> <a href="9.htm">9</a> <a href="10.htm">10</a> <a href="11.htm">11</a> <a href="12.htm">12</a> <a href="13.htm">13</a> <a href="14.htm">14</a> <a href="15.htm">15</a> <a href="16.htm">16</a> <a href="17.htm">17</a> <a href="18.htm">18</a> <a href="19.htm">19</a> <a href="20.htm">20</a> <a href="21.htm">21</a> <a href="22.htm">22</a> <a href="23.htm">23</a> <a href="24.htm">24</a> <a href="25.htm">25</a> <a href="notes.htm">Notes</a> <a href="HyperlinkedCSharpECMA.zip">Download</a><span class="ruler"></span><span class="heading">ECMA-334 C# Language Specification</span><span class="navigate"><a href="17.6.2.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="17.7.htm"><img src="next.gif" alt="next" border="0" /></a></span><span class="clause-depth"><a href="16.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="18.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.htm">17</a></span><span class="clause-title-previous"> Classes</span></span><span class="clause-depth"><a href="17.5.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.7.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.6.htm">17.6</a></span><span class="clause-title-previous"> Properties</span></span><span class="clause-depth"><a href="17.6.2.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.6.3.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">17.6.3</span><span class="clause-title"> Virtual, sealed, override, and abstract accessors</span></span><span class="locator">
Paragraph 1</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P1S1"></a>A virtual property declaration specifies that the accessors of the property are virtual.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P1S2"></a>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.</span> </span><span class="locator">
Paragraph 2</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P2S1"></a>An abstract property declaration specifies that the accessors of the property are virtual, but does not provide an actual implementation of the accessors.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P2S2"></a>Instead, non-abstract derived classes are required to provide their own implementation for the accessors by overriding the property.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P2S3"></a>Because an accessor for an abstract property declaration provides no actual implementation, its <span class="non-terminal"><a href="17.6.2.htm#accessor-body">accessor-body</a></span> simply consists of a semicolon.</span> </span><span class="locator">
Paragraph 3</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P3S1"></a>A property declaration that includes both the abstract and override modifiers specifies that the property is abstract and overrides a base property.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P3S2"></a>The accessors of such a property are also abstract.</span> </span><span class="locator">
Paragraph 4</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P4S1"></a>Abstract property declarations are only permitted in abstract classes (<a href="17.1.1.1.htm">§17.1.1.1</a>).</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P4S2"></a>The accessors of an inherited virtual property can be overridden in a derived class by including a property declaration that specifies an override directive.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P4S3"></a>This is known as an overriding property declaration.</span> <span class="sentence"><span class="sentence-number">4</span> <a name="P4S4"></a>An overriding property declaration does not declare a new property.</span> <span class="sentence"><span class="sentence-number">5</span> <a name="P4S5"></a>Instead, it simply specializes the implementations of the accessors of an existing virtual property.</span> </span><span class="locator">
Paragraph 5</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P5S1"></a>An overriding property declaration must specify the exact same accessibility modifiers, type, and name as the inherited property.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P5S2"></a>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.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P5S3"></a>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.</span> </span><span class="locator">
Paragraph 6</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P6S1"></a>An overriding property declaration may include the sealed modifier.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P6S2"></a>Use of this modifier prevents a derived class from further overriding the property.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P6S3"></a>The accessors of a sealed property are also sealed.</span> </span><span class="locator">
Paragraph 7</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P7S1"></a>Except for differences in declaration and invocation syntax, virtual, sealed, override, and abstract accessors behave exactly like virtual, sealed, override and abstract methods.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P7S2"></a>Specifically, the rules described in <a href="17.5.3.htm">§17.5.3</a>, <a href="17.5.4.htm">§17.5.4</a>, <a href="17.5.5.htm">§17.5.5</a>, and <a href="17.5.6.htm">§17.5.6</a> apply as if accessors were methods of a corresponding form: </span><ul><li><span class="sentence"><span class="sentence-number">3</span> <a name="P7S3"></a> A get accessor corresponds to a parameterless method with a return value of the property type and the same modifiers as the containing property.</span> </li><li><span class="sentence"><span class="sentence-number">4</span> <a name="P7S4"></a> A set accessor corresponds to a method with a single value parameter of the property type, a <span class="keyword">void</span> return type, and the same modifiers as the containing property.</span> </li></ul></span><span class="paragraph"><span class="example">[Example: In the example <pre class="code-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; }
}
</pre></span></span><span class="paragraph"><span class="example">X is a virtual read-only property, Y is a virtual read-write property, and Z is an abstract read-write property. </span></span><span class="paragraph"><span class="example">Because Z is abstract, the containing class A must also be declared abstract. </span></span><span class="paragraph"><span class="example">A class that derives from A is show below: <pre class="code-example">
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; }
}
}
</pre></span></span><span class="paragraph"><span class="example">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]</span> </span><span class="ruler"></span><table><tr><td><table align="left" bgcolor="navy"><tr bgcolor="navy"><td><font face="Arial,sans-serif" size="6" color="yellow"><strong>{ JSL }</strong></font></td></tr></table></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Jagger Software Ltd</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Company # 4070126</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>VAT # 762 5213 42</strong></font></td></tr></table><img src="valid-html401.png" align="left" height="31" width="88" alt="Valid HTML 4.01" /><img src="vcss.gif" align="left" height="31" width="88" alt="Valid CSS" /></body></html>⌨️ 快捷键说明
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