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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="17.5.5.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="17.5.7.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="17.4.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.6.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.5.htm">17.5</a></span><span class="clause-title-previous"> Methods</span></span><span class="clause-depth"><a href="17.5.5.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.5.7.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">17.5.6</span><span class="clause-title"> Abstract methods</span></span><span class="locator">
Paragraph 1</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P1S1"></a>When an instance method declaration includes an abstract modifier, that method is said to be an abstract method.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P1S2"></a>Although an abstract method is implicitly also a virtual method, it cannot have the modifier 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 method declaration introduces a new virtual method but does not provide an implementation of that method.</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 by overriding that method.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P2S3"></a>Because an abstract method provides no actual implementation, the <span class="non-terminal"><a href="17.5.htm#method-body">method-body</a></span> of an abstract method 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>Abstract method declarations are only permitted in abstract classes (<a href="17.1.1.1.htm">§17.1.1.1</a>).</span> </span><span class="paragraph"><span class="example">[Example: In the example <pre class="code-example">
public abstract class Shape
{
public abstract void Paint(Graphics g, Rectangle r);
}
public class Ellipse: Shape
{
public override void Paint(Graphics g, Rectangle r) {
g.DrawEllipse(r);
}
}
public class Box: Shape
{
public override void Paint(Graphics g, Rectangle r) {
g.DrawRect(r);
}
}
</pre>the Shape class defines the abstract notion of a geometrical shape object that can paint itself. The Paint method is abstract because there is no meaningful default implementation. The Ellipse and Box classes are concrete Shape implementations. Because these classes are non-abstract, they are required to override the Paint method and provide an actual implementation. end example]</span> </span><span class="locator">
Paragraph 4</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P4S1"></a>It is a compile-time error for a <span class="non-terminal"><a href="14.5.8.htm#base-access">base-access</a></span> (<a href="14.5.8.htm">§14.5.8</a>) to reference an abstract method.</span> <span class="example">[Example: In the example <pre class="code-example">
abstract class A
{
public abstract void F();
}
class B: A
{
public override void F() {
base.F(); // Error, base.F is abstract
}
}
</pre>a compile-time error is reported for the base.F() invocation because it references an abstract method. end example]</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 abstract method declaration is permitted to override a virtual method.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P5S2"></a>This allows an abstract class to force re-implementation of the method in derived classes, and makes the original implementation of the method unavailable.</span> <span class="example">[Example: In the example <pre class="code-example">
using System;
class A
{
public virtual void F() {
Console.WriteLine("A.F");
}
}
abstract class B: A
{
public abstract override void F();
}
class C: B
{
public override void F() {
Console.WriteLine("C.F");
}
}
</pre>class A declares a virtual method, class B overrides this method with an abstract method, and class C overrides that abstract method to provide its own implementation. 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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