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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="17.5.2.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="17.5.4.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="17.5.2.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.5.4.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">17.5.3</span><span class="clause-title"> Virtual 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 a virtual modifier, that method is said to be a virtual method.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P1S2"></a>When no virtual modifier is present, the method is said to be a non-virtual method.</span> </span><span class="locator">
Paragraph 2</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P2S1"></a>The implementation of a non-virtual method is invariant: The implementation is the same whether the method is invoked on an instance of the class in which it is declared or an instance of a derived class.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P2S2"></a>In contrast, the implementation of a virtual method can be superseded by derived classes.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P2S3"></a>The process of superseding the implementation of an inherited virtual method is known as overriding that method (<a href="17.5.4.htm">§17.5.4</a>).</span> </span><span class="locator">
Paragraph 3</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P3S1"></a>In a virtual method invocation, the run-time type of the instance for which that invocation takes place determines the actual method implementation to invoke.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P3S2"></a>In a non-virtual method invocation, the compile-time type of the instance is the determining factor.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P3S3"></a>In precise terms, when a method named N is invoked with an argument list A on an instance with a compile-time type C and a run-time type R (where R is either C or a class derived from C), the invocation is processed as follows: </span><ul><li><span class="sentence"><span class="sentence-number">4</span> <a name="P3S4"></a> First, overload resolution is applied to C, N, and A, to select a specific method M from the set of methods declared in and inherited by C.</span> <span class="sentence"><span class="sentence-number">5</span> <a name="P3S5"></a>This is described in <a href="14.5.5.1.htm">§14.5.5.1</a>.</span> </li><li><span class="sentence"><span class="sentence-number">6</span> <a name="P3S6"></a> Then, if M is a non-virtual method, M is invoked.</span> </li><li><span class="sentence"><span class="sentence-number">7</span> <a name="P3S7"></a> Otherwise, M is a virtual method, and the most derived implementation of M with respect to R is invoked.</span> </li></ul></span><span class="locator">
Paragraph 4</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P4S1"></a>For every virtual method declared in or inherited by a class, there exists a most derived implementation of the method with respect to that class.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P4S2"></a>The most derived implementation of a virtual method M with respect to a class R is determined as follows: </span><ul><li><span class="sentence"><span class="sentence-number">3</span> <a name="P4S3"></a> If R contains the introducing virtual declaration of M, then this is the most derived implementation of M.</span> </li><li><span class="sentence"><span class="sentence-number">4</span> <a name="P4S4"></a> Otherwise, if R contains an override of M, then this is the most derived implementation of M.</span> </li><li><span class="sentence"><span class="sentence-number">5</span> <a name="P4S5"></a> Otherwise, the most derived implementation of M is the same as that of the direct base class of R.</span> </li></ul></span><span class="paragraph"><span class="example">[Example: The following example illustrates the differences between virtual and non-virtual methods: <pre class="code-example">
using System;
class A
{
public void F() { Console.WriteLine("A.F"); }
public virtual void G() { Console.WriteLine("A.G"); }
}
class B: A
{
new public void F() { Console.WriteLine("B.F"); }
public override void G() { Console.WriteLine("B.G"); }
}
class Test
{
static void Main() {
B b = new B();
A a = b;
a.F();
b.F();
a.G();
b.G();
}
}
</pre></span></span><span class="paragraph"><span class="example">In the example, A introduces a non-virtual method F and a virtual method G. The class B introduces a new non-virtual method F, thus hiding the inherited F, and also overrides the inherited method G. The example produces the output: <pre class="code-example">
A.F
B.F
B.G
B.G
</pre></span></span><span class="paragraph"><span class="example">Notice that the statement a.G() invokes B.G, not A.G. This is because the run-time type of the instance (which is B), not the compile-time type of the instance (which is A), determines the actual method implementation to invoke. 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>Because methods are allowed to hide inherited methods, it is possible for a class to contain several virtual methods with the same signature.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P5S2"></a>This does not present an ambiguity problem, since all but the most derived method are hidden.</span> <span class="example">[Example: In the example <pre class="code-example">
using System;
class A
{
public virtual void F() { Console.WriteLine("A.F"); }
}
class B: A
{
public override void F() { Console.WriteLine("B.F"); }
}
class C: B
{
new public virtual void F() { Console.WriteLine("C.F"); }
}
class D: C
{
public override void F() { Console.WriteLine("D.F"); }
}
class Test
{
static void Main() {
D d = new D();
A a = d;
B b = d;
C c = d;
a.F();
b.F();
c.F();
d.F();
}
}
</pre>the C and D classes contain two virtual methods with the same signature: The one introduced by A and the one introduced by C. The method introduced by C hides the method inherited from A. Thus, the override declaration in D overrides the method introduced by C, and it is not possible for D to override the method introduced by A. The example produces the output: <pre class="code-example">
B.F
B.F
D.F
D.F
</pre></span></span><span class="paragraph"><span class="example">Note that it is possible to invoke the hidden virtual method by accessing an instance of D through a less derived type in which the method is not hidden. 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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